What Can I Put in My Engine to Run Smoother?

There's a certain kind of automotive frustration that's hard to put your finger on. The car starts fine. Nothing obvious is broken. But it doesn't feel quite right ,  there's a roughness at idle that wasn't there before, a slightly hesitant throttle response, a faint vibration that seems to have appeared from nowhere. The temptation is to assume the worst: something expensive must be wrong. Sometimes that's true. But sometimes, the answer is something you can buy at Repco for $30 and pour in yourself.

Additives, treatments, and fluid maintenance products have a complicated reputation. Some drivers swear by them. Others dismiss everything as clever marketing poured into a bottle of snake oil. The truth, as usual, sits somewhere in the middle. Some products are genuinely effective for specific problems. Others do very little. And a small number can actually cause harm if used on the wrong vehicle at the wrong time.

This guide cuts through the noise with honest, practical advice written specifically for Australian conditions ,  and tells you exactly what to pour in (or not), why it works, and when to bother.

Why Does a Car Start Feeling Rough Over Time?

Before you reach for any product, it helps to understand the common reasons a car gradually loses its smoothness. Most of them come down to one of three things: build-up, wear, or neglect

Carbon deposits: Every time fuel burns in a combustion chamber, it leaves trace amounts of carbon behind. Over thousands of kilometres, these deposits accumulate on fuel injectors, intake valves, piston tops, and combustion chamber walls. The result is reduced fuel atomisation (injectors can't spray as fine a mist when they're partially clogged), inconsistent combustion, and rough idle. In Australian conditions ,  where urban stop-start driving is the norm in Sydney, Melbourne, and Brisbane ,  this build-up accelerates compared to vehicles that regularly do long highway runs that burn deposits off naturally.

Oil sludge: Engine oil that's been left too long between changes, contaminated with moisture from short trips, or exposed to sustained extreme heat begins to break down and form sludge ,  a thick, tar-like material that restricts oil flow, clogs passages, and prevents critical components from being properly lubricated. In Queensland and the Northern Territory, where summer temperatures push underbonnet conditions well beyond what the original engineers planned for, sludge formation is an accelerated problem.

Worn or dirty injectors: Fuel injectors have incredibly fine nozzle holes that must atomise fuel into a precise, consistent mist. Contamination, varnish deposits, and wear over time cause these nozzles to spray unevenly ,  delivering too much fuel in one spot and too little in another. The result is misfires, rough idle, and fuel economy that slowly deteriorates without any single obvious moment of failure.

Tired coolant: Old coolant loses its anti-corrosion and heat-transfer properties over time. The result is a cooling system that doesn't maintain consistent temperature as well as it should, leading to minor hot spots and slightly inconsistent combustion ,  contributing to that 'not quite right' feeling that's hard to diagnose but very real.

What You Can Add ,  and What Each One Does

Here's the complete reference guide. These are the products with genuine track records for making a real difference:

Product Type	System	What It Does	Best For	How Often	Where To Buy (AU)
Engine oil flush	Oil system	Dissolves sludge, cleans internals, frees stuck rings	Rough idle, high mileage, unknown service history	Every 50,000–80,000 km or as needed	Repco, Supercheap, Burson
Oil stabiliser (e.g. Lucas)	Oil system	Thickens oil film, reduces consumption, quiets worn engines	High-mileage vehicle, oil burning, noisy valvetrain	Each oil change or as needed	Repco, Supercheap
Fuel system cleaner	Fuel & injectors	Cleans injectors, intake valves, combustion deposits	Rough idle, hesitation, poor economy	Every 10,000–15,000 km	Repco, Supercheap, Ampol
Ceramic friction modifier (e.g. Liqui Moly Cera Tec)	Oil system	Micro-ceramic coating reduces metal-to-metal friction	Any vehicle wanting protection, especially older ones	Each oil change or annually	Auto parts stores, online
Diesel fuel additive	Fuel (diesel only)	Cleans injectors, boosts cetane, removes moisture	Diesel rough idle, hard starting, smoky exhaust	Each full tank or monthly	Repco, Supercheap
Octane booster (petrol)	Fuel (petrol only)	Raises octane rating, reduces knock	Knock/ping under load, older engines	As needed or per tank	Auto parts stores
Engine decarb / top-end cleaner	Fuel & intake	Cleans carbon from valves, injectors, combustion chamber	Poor performance, high carbon buildup	Every 30,000–50,000 km	Workshop application
Coolant flush additive	Cooling system	Cleans scale, corrosion, old coolant breakdown products	Overheating, old coolant, temperature fluctuations	Every 2–3 years	Repco, Burson

* All prices and availability are approximate as of April 2026. Always check compatibility with your vehicle before use. Follow manufacturer dosage instructions carefully.

Oil Treatments ,  The Most Impactful Category

Of everything on the table above, oil-side treatments typically have the most noticeable effect on how a car feels ,  because oil is in contact with every critical moving part inside the engine. The quality, cleanliness, and viscosity of your oil affects noise, smoothness, fuel economy, and long-term wear all at once.

Engine Oil Flush ,  For High-Mileage and Unknown-History Vehicles

An engine flush is a chemical additive poured into the oil filler port before an oil change. The engine idles for 10–30 minutes (depending on the product), the flush circulates through the oiling system dissolving sludge, varnish, and carbon deposits, and then you drain the oil ,  taking all of that contamination out with it. You immediately refill with fresh oil and a new filter.

Products like Penrite Engine Flush, Liqui Moly Engine Flush, and Repco-branded flush concentrates are widely available across Australian auto parts stores. Australian Land Cruiser and D-Max owners, in particular, frequently swear by engine flush products before switching oil grades or after extended service intervals ,  the results are often dramatic when the old oil is jet black even after relatively low kilometres.

The genuine benefits of an engine flush include:

•       Restored oil flow to tight passages and VVT (Variable Valve Timing) solenoids that have been partially blocked by varnish

•       Freed piston rings ,  rings that have become gummed with deposits can stick, reducing compression and causing blow-by

•       Cleaner start conditions for fresh oil ,  new oil in a dirty engine gets contaminated almost immediately

•       Noticeably quieter valve train in many vehicles after the flush removes the varnish restricting oil to hydraulic lifters

The important cautions:

•       On very high mileage engines with a history of poor maintenance, sludge may be acting as a makeshift seal over worn gaskets or cracked seals. Dissolving that sludge can expose leaks that were previously hidden ,  the flush didn't cause the leak, but it revealed it.

•       Don't leave the flush product in the engine for longer than the instructions specify. It's designed to dissolve deposits ,  leaving it in significantly longer than directed is not beneficial.

•       Always replace the oil filter immediately after the flush. All those freed deposits need to be captured ,  and an old, saturated filter won't do that.

Oil Stabilisers ,  For Noisy or High-Mileage Engines

If an engine has started making valve train noise ,  the classic 'tick-tick-tick' at idle that quietens once the car warms up ,  or if you're noticing that oil consumption has increased, an

Products like Lucas Oil Stabiliser and Penrite's Morey's Oil Stabiliser work by increasing the viscosity and film strength of the oil. They help the oil cling to metal surfaces more tenaciously, particularly helpful in engines where worn clearances allow oil to drain off bearing surfaces during cold starts ,  the primary cause of that morning valve tick.

For a V8 HiLux, an older Commodore, or any vehicle approaching 200,000 km, a good oil stabiliser can make a noticeable difference to cold-start noise within the first few minutes of the next drive after it's added. It won't fix worn bearings or valvetrain components that genuinely need mechanical attention ,  but it buys time and comfort in the interim.

One Important Warning on Oil Additives

Modern engine oils already contain a carefully formulated additive package designed by the oil manufacturer. Adding too much of an aftermarket oil additive can upset this balance ,  particularly if you add multiple products simultaneously. Stick to one treatment at a time, follow dosage instructions, and never overfill your engine beyond the maximum mark on the dipstick. More is not better with oil additives.

Ceramic Friction Modifiers ,  For Those Who Want Protection

Products like Liqui Moly Cera Tec represent a different approach: rather than cleaning or thickening the oil, ceramic friction modifiers deposit microscopic ceramic particles (typically molybdenum disulfide or silicon carbide compounds) onto metal surfaces inside the engine. These particles fill microscopic surface irregularities in the metal, creating a smoother, harder surface that reduces friction directly.

The effect is subtle but real: reduced friction means the engine runs more freely, produces more of its available power at the wheels, generates less heat, and experiences less wear. For a driver who simply wants their car to feel sharper and more refined, a ceramic friction modifier added at each oil change is one of the most genuinely worthwhile treatments available ,  particularly for vehicles with high-revving engines or those doing a lot of motorway kilometres.

Fuel Treatments ,  Cleaning From the Inside Out

Fuel system cleaners address a different set of problems to oil treatments. Where oil treatments target the internal mechanical components of the engine, fuel treatments target everything the fuel touches: injectors, intake valves, combustion chambers, and fuel delivery components.

Fuel Injector and System Cleaners

This is the category with the strongest real-world evidence of effectiveness ,  and the most relevant for Australian urban drivers. Every kilometre of stop-start city driving deposits small amounts of varnish and carbon on injector nozzles and, in direct-injection engines, on the backs of intake valves (which are no longer washed by fuel as they were in older port-injected designs).

A quality fuel system cleaner ,  Liqui Moly Injection Cleaner, Wynns Injector Cleaner, or STP Complete Fuel System Cleaner ,  added to a fuel tank every 10,000–15,000 km works through the fuel delivery system and combustion process to dissolve and remove these deposits. The result is typically:

•       Smoother, more consistent idle ,  particularly noticeable if the idle had become slightly rough or hunting

•       Improved throttle response ,  the hesitation between pressing the accelerator and getting a clean power response often improves

•       Better fuel economy ,  clean injectors atomise fuel more efficiently, getting more energy from each litre

•       Reduced emissions ,  more complete combustion means less unburnt hydrocarbons in the exhaust

For diesel vehicles specifically, dedicated diesel fuel additives serve an additional purpose: they boost cetane rating (the diesel equivalent of octane for petrol ,  a higher cetane number means faster, cleaner ignition), remove moisture from the fuel system, and lubricate the high-pressure injection pump. Australian diesel vehicles ,  particularly the ubiquitous Toyota HiLux, Ford Ranger, Mazda BT-50, and Isuzu D-Max ,  are excellent candidates for regular diesel fuel treatment given the demands of towing, varying fuel quality between city servo and outback roadhouse, and the sustained heat that accelerates water contamination in diesel tanks.

Octane Boosters for Petrol Engines

Octane boosters get a mixed reception, and honestly that's fair ,  many of the cheap ones do very little. The physics is simple: raising the octane rating of fuel prevents knock (detonation), which allows the engine control unit to run more aggressive ignition timing, which produces more power and smoother combustion. But most over-the-counter octane boosters raise the octane rating by only 0.5 to 1.5 points ,  not enough to make a meaningful difference in most street cars.

Where they genuinely have value is in turbocharged or performance vehicles that run on premium unleaded (95 or 98 RON) and are being used in demanding conditions ,  towing, high ambient temperatures, or track days. In these situations, a quality octane booster like Torco Unleaded Accelerator can provide enough additional knock resistance to prevent the ECU from pulling ignition timing under load, keeping power and smoothness where they should be.

Don't Forget the Cooling System

The cooling system is one of the most overlooked contributors to how smoothly and consistently a car runs. An engine that doesn't maintain a stable operating temperature produces inconsistent combustion ,  the ECU is constantly compensating for temperature variations, injector pulse widths are changing, and the overall running quality is subtly rougher than a car with a clean, efficient cooling system.

Over time, the coolant itself degrades. The corrosion inhibitors break down, scale and deposits form on the inside of aluminium components, and the coolant's ability to transfer heat efficiently diminishes. In Australian conditions ,  where summer puts significant thermal stress on cooling systems ,  degraded coolant contributes to more variable operating temperatures and, in some cases, the kind of subtle overheating that causes head gaskets to fail over time.

The fix is straightforward: a coolant system flush (using a product like Penrite Cooling System Flush or Nulon Radiator Flush) removes the old, contaminated coolant along with the scale and deposits, and prepares the system for fresh coolant that actually protects the way it's supposed to. For most Australian vehicles, coolant should be replaced every 2–3 years ,  though some long-life formulations can go longer. If you don't know when yours was last changed, treat it as due.

What Additives Won't Fix ,  Being Honest About the Limits

There's a temptation to reach for a bottle every time a car doesn't feel right. Sometimes that's the right move. But additives have real limits, and being clear about those limits saves money and prevents disappointment.

Worn mechanical components: If the engine is rough because of genuinely worn bearings, a cracked or leaking head gasket, compression loss from damaged piston rings or valve seats, or a failing VVT actuator ,  no additive will fix these. They might temporarily mask some symptoms, but they won't address the root cause. Rough idle caused by mechanical failure needs mechanical repair.

Misfires from ignition system faults: A rough idle caused by failing spark plugs, a damaged ignition coil, or a deteriorated plug lead will not be improved by any oil or fuel treatment. Spark plugs should be replaced on schedule (every 30,000–40,000 km for conventional plugs, every 80,000–100,000 km for iridium/platinum plugs). Worn plugs are one of the most common causes of rough idle and hesitation ,  and at $80–$150 for a set, they're far cheaper than people assume.

Vacuum leaks: An air leak in the intake system creates a lean fuel mixture that causes rough idle and hesitation. No additive addresses a physical air leak. These are found by a mechanic with a smoke machine and fixed by replacing the leaking hose, gasket, or throttle body seal.

Fuel pressure problems: A weak fuel pump or clogged fuel filter produces lean conditions under load that feel like rough running. Fuel treatments help injectors but won't restore a failing fuel pump's ability to deliver adequate pressure.

Quick Diagnostic: What's Causing Your Rough Running?

→  Rough idle that's WORSE when cold → Oil treatment or flush may help (sludge restricting oil to lifters)

→  Rough idle that's WORSE when warm → Likely fuel/injector issue ,  try fuel system cleaner first

→  Hesitation on acceleration → Fuel injector cleaner; check spark plugs if no improvement

→  Rough idle + poor economy together → Strong indicator of injector/combustion deposit build-up

→  Tick-tick-tick noise at startup → Oil stabiliser and fresh oil change ,  likely valve train starved at cold start

→  Rough idle + check engine light + misfire code → Spark plugs or ignition coils first ,  not an additive problem

→  Engine running hot / temperature fluctuating → Coolant flush, not an oil treatment

→  High oil consumption (topping up between changes) → Oil stabiliser + compression test to rule out mechanical cause

Why Australian Conditions Make This More Important

Most automotive products are designed and tested in European or American conditions. Australian conditions are harsher in specific ways that make engine maintenance more ,  not less ,  important:

Extreme heat: Darwin, Alice Springs, Broken Hill, Kalgoorlie, and most of inland Queensland regularly see ambient temperatures above 40°C. At these temperatures, underbonnet conditions push lubricants well beyond their design range. Oil degrades faster, sludge forms more readily, and additives that restore oil film strength under heat become genuinely important rather than optional.

Stop-start urban driving: Sydney, Melbourne, and Brisbane are notorious for urban driving patterns that are extremely hard on engines ,  short trips that never fully warm the oil, repeated cold starts, and long periods of idling in traffic. This driving pattern accelerates sludge formation and prevents the natural 'self-cleaning' that occurs when an engine reaches and sustains full operating temperature over a long highway run.

Dusty outback conditions: Vehicles that do any driving on unsealed roads face dramatically higher levels of dust and particulate contamination entering the engine through the air intake. Even with regular air filter changes, some dust inevitably gets through ,  and that means more abrasive particles in the oil, faster bearing wear, and more rapid degradation of the additive package in the oil.

Variable fuel quality: Fuel quality in major Australian cities is good. But fuel quality at remote roadhouses ,  where tanks may sit for weeks before a tanker refills them, and where high temperatures cause fuel degradation ,  is sometimes significantly worse. This is particularly relevant for diesel vehicles travelling between major centres on the Nullarbor or through outback Queensland. A diesel fuel treatment with moisture dispersant and cetane boost becomes more than a performance upgrade; it's practical protection against variable fuel quality.

Frequently Asked Questions

Will an engine flush damage my car?

For most modern, well-maintained vehicles, an engine flush performed correctly with a quality product will not cause damage. The concern you sometimes read about ,  that a flush will 'loosen chunks of sludge that clog the engine' ,  is a risk that's been somewhat overstated for modern, detergent-based flush products. The scenario where a flush causes problems is specific: in very old, high-mileage engines that have been poorly maintained for a long time, heavy sludge deposits may be masking worn or cracked seals and gaskets. When the flush dissolves the sludge, the underlying damage is revealed ,  but the flush didn't cause it, it just made it visible. For a regularly serviced vehicle with known maintenance history, an engine flush is a safe and beneficial treatment. For a vehicle with an unknown history or very high mileage with suspected chronic neglect, discuss it with a trusted mechanic first ,  they can advise whether the vehicle is a good candidate based on what they know about its condition.

How often should I use a fuel injector cleaner?

For most Australian drivers doing urban driving, every 10,000–15,000 km is a sensible interval. This roughly coincides with a standard service interval, making it easy to add to your maintenance routine. If you do predominantly highway driving with long, sustained runs, your engine naturally runs hotter and burns off deposits more efficiently ,  you might find 20,000 km works perfectly well. Conversely, if you do a lot of very short trips (under 10 km), the injector deposits build faster and a tighter interval makes sense. The practical trigger is simple: if your idle feels slightly rough, throttle response feels slightly dull, or your fuel economy has quietly declined without any other obvious cause, that's a good time to add a fuel system cleaner regardless of the kilometre count. Many Australian drivers add it to every second or third tank of fuel as ongoing maintenance, which is also a reasonable approach for older or higher-mileage vehicles.

Is it safe to mix different additives?

Generally, no ,  and this is one of the more important rules to follow. Modern engine oils contain a sophisticated, carefully balanced additive package. Adding multiple aftermarket treatments simultaneously risks disrupting that chemical balance in ways that can actually reduce protection rather than increase it. The practical rule is: use one additive product at a time, assess the result over a few hundred kilometres, and only add another treatment if needed for a specific additional purpose. Never mix an oil flush product with an oil stabiliser ,  the flush is designed to thin and dissolve deposits while the stabiliser is designed to thicken ,  these work against each other. Similarly, don't combine multiple fuel treatments in the same tank unless the product instructions specifically say it's safe to do so. The exception is that most fuel system cleaners can be safely combined with a diesel fuel additive if one is formulated for petrol and the other specifically for diesel ,  but only if you're using the correct one for your fuel type, obviously.

The Bottom Line

A rougher, less responsive car doesn't always need an expensive workshop visit. In many cases, the right additive at the right time genuinely makes a noticeable difference ,  a fuel system cleaner restoring idle quality, an oil flush clearing years of sludge and giving the engine a fresh start, or an oil stabiliser quieting a noisy valvetrain on a high-mileage vehicle.

The key is matching the treatment to the actual problem. Oil treatments address lubrication, wear, and sludge. Fuel treatments address injector deposits, combustion quality, and fuel delivery. Coolant treatments address temperature regulation. None of them fix mechanical failures ,  but for gradual performance decline caused by deposits, contamination, and age, they're often exactly the right tool.

In Australian conditions ,  with the heat, the stop-start urban driving, the outback dust, and the variable fuel quality ,  this kind of proactive maintenance matters more than in most markets. A bottle of something from Repco every 10,000 km is a far cheaper insurance policy than the repairs that neglect eventually demands.