Thin oil is the internet's favourite explanation for modern engine wear. There is a controlled study behind the concern, and it found the thinner oil wore more in about two thirds of the operating conditions tested - and less in the rest. The largest wear variable in the whole programme was not the oil grade at all.
- Applies to
- Petrol engines specifying SAE 0W-16 or 0W-20
- Evidence
- Radionuclide tracer wear study, one turbo GDI engine
- Direction
- Higher wear in ~2/3 of conditions, lower in the rest
- Bigger variable
- Cold start, on every component measured
- Current standard
- ILSAC GF-7A and GF-7B, effective 31 March 2025
The number on the bottle is not the number that matters
A multigrade oil is defined by three things under SAE J300: maximum cold cranking and pumping viscosities, a kinematic viscosity band at 100°C, and a minimum high-temperature/high-shear (HTHS) viscosity measured at 150°C and one million reciprocal seconds, as set out in API 1509. HTHS is the figure measured at loaded-bearing shear rates, which is why manufacturer and ACEA specifications are written around it rather than around the grade label.
That reframes the size of the step. API 1509 records HTHS minimums of 2.3 mPa·s for an SAE 16 grade, 2.6 for a 20, 2.9 for a 30 and 3.5 for a 40. Going from 0W-20 to 0W-16 gives up about 12% of guaranteed film-forming viscosity, not the 20% the numbers imply.
The widely repeated claim that a 5W-30 gives a 50% thicker film than a 0W-20 is one we could not trace to any primary source. The defensible comparison is HTHS minimums, 2.9 against 2.6 - about 11.5%.
The first number does not enter into it. A 0W-16 and a 0W-30 must meet identical cold cranking and pumping limits, an MRV cap of 40,000 cP and a gelation index of 12, under the same specification tables. So "thin oil flows better at start-up" and "going up a grade hurts cold starting" are both wrong, for the same reason.
What the controlled wear study measured
We found one study that isolates viscosity properly: the CRC AVFL-28 Phase II programme run by Southwest Research Institute, published October 2019. It ran an SAE 5W-30 (12.346 cSt at 100°C) and an SAE 0W-16 (7.066 cSt) built on the identical additive package through a Ford 2.0L EcoBoost, with rings, liner, rod and main bearings, balance shaft bearings and turbo components made radioactive so wear could be measured continuously rather than inferred from a teardown.
The headline finding is the one people quote: the report states that the "lower viscosity lubricant resulted in higher wear across roughly two thirds of the engine operating conditions", in its own words.
Two caveats belong with it. It is one turbocharged direct-injection engine family on certified test fuel, so it does not license a conclusion about naturally aspirated or port-injected engines. And the report itself was scoped to inform future test development, not to rank oils on a shelf.
The evidence that cuts the other way
In the remaining third of conditions, the thicker oil wore more. On the crankshaft main bearings, four of the eight reported sequences gave higher wear on the 5W-30, and three of those four showed no measurable wear at all on the 0W-16. On the top ring face it was the 5W-30 that produced the high wear. The report says so plainly: "Although it may be expected that the SAE 0W-16 lubricant would give higher wear across all operating conditions and engine components tested, this did not prove to be the case" - its own conclusion.
The standards also went the opposite way to the folklore. ILSAC GF-6 added three tests GF-5 did not have, and GF-7 then tightened the timing-chain wear limit from 0.085% to 0.080% elongation and added an aged-oil pre-ignition requirement, per the API 1509 tables. GF-7B, the 0W-16 category, is held to the same valvetrain wear limits as GF-7A holds 0W-20 through 10W-30: 2.7 mm³ average intake lifter volume loss and 400 ppm end-of-test iron.
Two more inconvenient facts. The committee writing those standards, ILSAC, is Ford, General Motors, Stellantis and the Japan Automobile Manufacturers Association - the parties who pay for engine failures under warranty, not oil companies. And a heavy-duty fleet test of low-viscosity oils concluded that with a correct formulation there was no significant difference in engine wear, oil consumption or HTHS variation. That is a different service duty and should be read as such, but it points the same way: formulation, not viscosity alone.
Cold starting outranked the grade
The same study found that "the cold start cycles operated at the beginning of every day of testing showed the highest wear rates of any of the engine test cycles" - again, its words. Cold start was also the condition that produced measurable wear on every irradiated component at once, with the turbo shaft, top ring face and top ring side responding hardest.
Fuel dilution belongs in the same paragraph, because what ends up in the sump is not always what came out of the bottle. Testing on a highly boosted direct-injection petrol engine measured crankcase fuel dilution up to 9% on a cyclic load cycle simulating customer driving, "causing significant drop in the oil viscosity" (Hu, Teng, Luo and Chen, SAE 2015-01-0967). That study traces it to long injection durations at high-speed-end torque putting liquid fuel on the piston, so it is a duty-cycle effect rather than an automatic property of direct injection. American Honda addressed dilution on 1.5L turbocharged engines in a service bulletin titled "Understanding Oil Dilution", filed with US regulators as document MC-10152402 dated 14 January 2019, with a remedy of software updates and an oil change.
A car that does nothing but short cold trips accumulates the highest-wear condition in the study on every single journey, and no grade on the shelf corrects that.
Where the concern has the most to stand on
The strongest honest version of the worry is not "thin oil is bad" but "thin oil in hardware not built around it". Reviewing the friction literature, Lee and Zhmud write that to maintain hydrodynamic lubrication at lower viscosity, "substantial modifications in the engine hardware are often required including surface finish specifications, bearings, filtration systems, and oil pump, galleries and squirters", and that without them "the risk of excessive wear is real and cannot be ignored" (Lube-Tech 133).
The same review reports that the fuel saving is not linear either: simulation of a modern engine showed the benefit of moving from 0W-20 to 0W-8 concentrated at medium-to-high speed and low load, while the low-rpm, high-load corner showed both worse economy and elevated main-bearing wear risk. On an older cast-iron-bore engine with bucket tappets, the lowest-viscosity oil gave the highest friction at low rpm.
That corner - lugging near the maximum torque curve at low road speed - is exactly what a loaded caravan on a grade produces, which is why towing is the Australian version of this question. The AVFL-28 test matrix does treat towing as severe duty, with a hot trailer-tow cycle and a deliberately harsher variant at 115°C oil, raised 5 to 10°C above the standard tow cycle with intake air 20°C higher. Read that 115°C for what it is: a dynamometer severity setpoint, not a measurement of what a towing vehicle's oil reaches in service. That report publishes no field oil temperatures, and neither could we find any. Owner-reported sump temperatures on forums are not evidence.
What your manual says, and what we could not verify
Manuals are more permissive than the argument assumes, and more conditional than the quotes circulating suggest. Toyota's US owner's manual for the 2023 Corolla with the 2.0L M20A-FKS specifies JASO GLV-1, SAE 0W-8, and calls it "the best choice for good fuel economy and good starting in cold weather"; it permits SAE 0W-16 when 0W-8 is unavailable but requires that "it must be replaced with SAE 0W-8 at the next oil change" (OM02568U, maintenance data). The frequently quoted line about higher viscosity suiting "high speeds, or under extreme load conditions" sits in that manual's explanation of what the second number in the grade means, not in a recommendation to depart from the specified oil. That document states it applies to US vehicles only.
Four popular claims we could not stand up:
- That thin oil shortens engine life. No source we reached compares mileage to failure between grades. A wear rate measured on a dynamometer is not a life expectancy.
- That a grade step up damages variable valve timing. We could find no manufacturer document, service bulletin or technical paper showing phaser damage from one grade up in a healthy engine.
- That thin oil causes low-speed pre-ignition. The evidence points at detergent chemistry: one controlled study found lower pre-ignition rates with the lower-viscosity oil at high magnesium concentrations, and the industry fix was cutting calcium, not thickening oil. Pre-ignition is also a turbocharged direct-injection event, and not the same thing as wear.
- That Australian-delivered cars get a thicker specification. We could not open an Australian-market manual serving its oil-selection page to confirm it.
One thing that is genuinely local: Australia's New Vehicle Efficiency Standard commenced on 1 January 2025 with supplier obligations from 1 July 2025, so a fraction of a percent of fuel economy now carries a dollar value to an importer here for the first time. And under Australian Consumer Law, consumer guarantees are not lost by using an independent repairer, as the ACCC sets out - use an oil meeting your manual's grade and approval, from whoever you like, and keep the invoice showing the product used.