Most engine oil additives on the shelf promise to cut friction, reduce wear and extend engine life. Some cost £15, some cost £50, and most make identical claims. Before buying any of them, it’s worth knowing that some have been independently tested and found to cause more harm than good.
The answer is not the same for every type of engine oil additives. A few work. Several don’t. And at least one category has been shown to actually increase engine wear while reducing friction — which sounds contradictory until you understand what’s happening inside the engine.
What’s Already Inside Your Engine Oil
Your engine oil already has additives in it. Every bottle of fully synthetic — ACEA C3, API SP, whatever spec your car needs — comes with a chemical package built in at the factory.
The detergents in your engine oil keep the inside of the engine clean — they lift carbon and soot off metal surfaces and carry it to the oil filter. The antioxidants stop the oil breaking down from the heat the engine produces. Viscosity index improvers stop the oil thinning too much when the engine gets hot, or thickening too much on a cold morning. Anti-wear additives — mainly a compound called ZDDP — form a protective film on metal surfaces when they come under load.
These engine oil additives were put there by engineers who spent years getting the ratios right for that specific oil. Adding something on top of a fully formulated oil changes that balance — which is exactly why some engine oil additives work and others cause damage.
PTFE Engine Oil Additives — The Ones That Can Damage Your Engine
PTFE is the chemical name for Teflon — the non-stick coating on frying pans. Same stuff. Slick 50 was the most famous PTFE-based engine oil additive, marketed aggressively for years on the claim that it coated engine parts and dramatically reduced friction.
DuPont — the company that invented Teflon — went on record stating that PTFE is not useful as an ingredient in oil additives for internal combustion engines. NASA’s Lewis Research Center tested PTFE-based engine oil additives and found no benefit to bearing surfaces. In some tests they found the opposite — PTFE solid particles accumulated at oil passage inlets and blocked oil from reaching the components it was supposed to protect.
The University of Utah Engineering Experiment Station ran tests on a Chevrolet six-cylinder engine using a PTFE additive. Friction went down 13.1 percent — the kind of number additive companies love putting on packaging. But the same test found iron contamination in the oil doubled after treatment. Iron contamination means metal is being worn off engine parts. Friction went down, wear went up.
In 1997 the Federal Trade Commission charged Quaker State — the maker of Slick 50 — with making false and unsubstantiated advertising claims. They settled. Slick 50 later removed PTFE from its main formula.
PTFE engine oil additives are the clearest example of engine oil additives that can hurt rather than help. Avoid them.
Also Read: The Best Diesel Engine Oil for Your Car: Castrol vs Shell vs Liqui Moly vs Mobil
ZDDP Boosters — Work, But Only If Your Engine Needs One
ZDDP — zinc dialkyldithiophosphate — is already present in modern engine oils. It forms a protective layer on metal surfaces under high pressure, specifically on camshaft lobes and valve train components. It’s been in engine oil for over 70 years and the protection it provides is well documented.
The problem is that modern engine oils contain less ZDDP than older formulations. Post-2004 API specifications reduced phosphorus limits to protect catalytic converters — phosphorus from ZDDP can reduce catalyst efficiency over time. A 2001 engine oil might have carried 1,200 parts per million of ZDDP. A current API SP oil carries around 600 to 800 ppm.
For most modern engines with roller followers this doesn’t matter — roller followers don’t put the same load on the camshaft as flat tappets do. For older engines built before the late 1980s — classic cars, older American V8s, engines with flat tappet camshafts — the reduced ZDDP in modern oil is a real concern. These engines were designed around higher zinc and phosphorus levels.
A ZDDP booster additive makes sense for a flat tappet engine running modern oil. Products like Driven Racing Oil’s zinc additive or Idemitsu’s ZDDP additive add back the protection level those older engines were designed for. Used in the right application, these engine oil additives have genuine scientific support behind them.
For a modern engine with a catalytic converter — skip the ZDDP booster. The oil already has the right amount. Adding more phosphorus can damage the catalyst.
Should You Use an Engine Flush Additive?
Engine flush products are poured into the oil before a change, the engine is run for 10 to 15 minutes, then the old oil and flush are drained together. The idea is to clean out sludge and deposits that have built up over years of service.
Change your oil on time every time and there’s nothing to flush — the oil has been doing the cleaning job already. Buy a car with unknown service history or one that hasn’t seen an oil change in years, and a flush can shift the sludge that’s built up in places the fresh oil can’t reach on its own.
Aggressive flush products dislodge deposits too quickly. Chunks break off and block oil passages before they reach the filter. Stick to a gentle detergent-based flush, not solvent. Minimum run time on the instructions. Fresh oil in immediately after. Engine stays at idle the whole time.
These engine oil additives can be useful — but the risk of getting it wrong is higher than with other additive types.
Stop Leak Additives — Temporary Fix, Not a Solution
Stop leak engine oil additives work by swelling rubber seals and gaskets to reduce or stop oil seeping past them. They don’t repair a seal. They swell it. Most contain petroleum-based solvents or seal conditioners like esters.
For a minor weep from an aged seal — a small damp patch after parking, not a puddle — a stop leak additive can buy time before a proper repair. For a seal that has physically torn or failed, it does nothing.
Rubber seals aren’t meant to stay swollen permanently. Keep using stop leak engine oil additives long term and the seal degrades faster than it would have without treatment — a slow drip becomes a proper leak sooner.
Use stop leak additives as a short-term measure only, not as a permanent solution.
Molybdenum and Friction Modifier Engine Oil Additives
Molybdenum disulfide — MoS2 — and organic friction modifiers are a different story from PTFE. These compounds work at a molecular level, forming a thin layer on metal surfaces that genuinely reduces friction under boundary lubrication conditions. This is the state where the oil film between moving parts is at its thinnest — typically at startup, low speeds and high loads.
University labs in the UK, Japan and Germany have tested molybdenum-based friction modifiers. The results are consistent — they reduce metal-to-metal contact when the oil film is thinnest, which is during cold starts and low-speed driving. Peer-reviewed research, not a manufacturer claim.
Some premium engine oils already include friction modifiers in their formulation — Mobil 1 and Castrol Edge both use them. If your oil already has them, an aftermarket friction modifier additive adds little. If your oil doesn’t, a quality molybdenum-based additive like Liqui Moly Cera Tec or Millers Nanodrive can provide genuine friction reduction.
These engine oil additives are the most scientifically supported of the aftermarket options.
Engine Oil Additives — Which Ones Work and Which Ones Don’t
| Engine Oil Additive Type | Does It Work | Risk Level |
|---|---|---|
| PTFE / Teflon based | No — can increase wear | High — avoid |
| ZDDP booster | Yes — for flat tappet engines only | Low if used correctly |
| Engine flush | Sometimes — for neglected engines | Medium — use carefully |
| Stop leak | Temporarily — minor seal weeps only | Medium — short term only |
| Friction modifier (MoS2) | Yes — measurable friction reduction | Low |
FAQ
Do engine oil additives actually work?
Depends which type. Friction modifiers based on molybdenum have solid scientific support. PTFE-based additives have been shown to increase engine wear in independent tests. ZDDP boosters work for older flat tappet engines. Most modern engines running quality oil don’t need any aftermarket engine oil additives.
Can engine oil additives damage my engine?
PTFE-based engine oil additives have been shown to clog oil passages and increase iron contamination in independent tests. NASA’s Lewis Research Center found detrimental effects in bearing surface tests. DuPont stated PTFE is not appropriate for internal combustion engines.
Should I use a ZDDP additive in my modern car?
No. Modern engines with roller followers and catalytic converters don’t need extra ZDDP. Adding it can damage the catalytic converter by increasing phosphorus levels above the threshold the converter was designed for.
Are engine flush additives safe?
For a well-maintained engine — unnecessary. For a neglected engine with sludge buildup — can be useful if used carefully with a gentle detergent-based product and minimum run time. Don’t use solvent-based flushes or run the engine hard during the flush.
What engine oil additive is actually worth buying?
For a classic or older vehicle with a flat tappet camshaft — a ZDDP booster. For any engine — a quality oil that already includes friction modifiers and proper additive chemistry is better than adding aftermarket engine oil additives to a cheap oil.
Was Slick 50 ever proven to make false claims?
Yes — the FTC charged Quaker State in 1997. The ads said Slick 50 reduced engine wear by up to 50 percent. Quaker State settled the case and paid out. The PTFE formula disappeared from the product shortly after.
