Can thicker oil cause more wear?

Feb 6, 2021
Yes, another thick vs thin thread, but this one's a little different. Let's take my car for example (just to make it simple), it specifies a 5w-20, 5w-30 or 10w-30. Let say I decide to run a 5w-40 or even just for discussions sake, a 15w-50 oil for the life of the engine. No VVT codes or other strange things happen, and the vehicle is always started in warm weather (we'll say no colder than 72°F). Is there any mechanical or scientific reason that running a thicker oil could cause increased wear?
Only time a thicker oil could cause more wear is if it's too thick to fully pump at cold start-up. Therefore, always use an appropriate "W" (Winter) rating for the coldest start-ups. SAE came up with the "W" rating in J300 for a reason. If I lived where it got super cold (like -25F), I'd probably run a 0W-30 to ensure good cold start-up pumpability and flow.
Only if its extremely thick but increasing the hths by <1cst isn't much. Going with a much thicker oil in some cases will lead to no increase in wear protection anymore just loss of fuel economy. But it has been shown that going from 16 and 20 to 30 or 30 to 40 can decrease wear. Now if you pit a crap 10w-40 up against some very stout 0w-16 then yeah the 0w-16 may reduce wear but if I had to put supertech syn blend in my engine id probably go 1 or 2 grades up. If the base oil quality and anti wear package quantity is equal just going a bit thicker has been proven to reduce wear for most people and by that I mean a normal person's driving style like a lot of stop and go acceleration which should have a bit thicker film. Highway wear on cruise control isn't the same but the test procedures typically have static operating conditions which can show differences but not the big picture
Thicker oil should provide better moft and thicker film and I assume less chance of contact and wear especially in summer heat and with high revving engines or high load or high rpm or hot running turbo engines.

I always use one grade over the manufacturer recommendation (to neutralize cafe) or at least use a higher hths (a3/b4) of the recommended oil if possible. Meaning I never use xW-20 or lower. I prefer a solid 30 or a high hths 30 and/or a regular 40. However I pay good attention to the W rating as @ZeeOSix mentioned. In general, I prefer narrow spread oils but have been violating my rule lately by using some Euro 0W-40 for the summer.
Running 15w-50 in an engine which a 5w-20, 5w-30 or 10w-30 are specified cannot be a good thing for an engine during start up especially in colder temperatures.
Not a typical scenario, but I've seen this happen more than once before.

Drag racing engines, particularly OEM blocks with unmodified oil passages, using stock/low oil capacity with a high volume oil pump and a viscous oil like 20w-50 with oil temp that's just 120°F because it's not running long enough to heat up.

On the top end of the track, at high rpm, the driver notices the oil pressure has gone from 70 psi to 0 psi. The breathers in the valve covers are soaked in oil. Two seconds after letting off the throttle, the oil pressure goes back to near 70 psi. The oil pump is cavitating because it's pumping more volume than the viscous (100+ cSt at that temp) oil can drain back to the pan. All of the stock sump's capacity is in the heads and lifter valley.

Related to that, I've seen quite a few snapped oil pump drives in older Ford small blocks using a viscous 15w-40 to 20w-50 oil in cold temperatures. Usually the idiot owner/user cranking it up at <32°F and proceeding to rev it to the moon. They usually don't know the consequences of their carelessness until they hear the rod knock.
My answer is based on four things; generalizations about design, use, state of tune and environment are the inputs I'd consider here.

- presuming the vehicle/equipment doesn't inherently have an OEM design flaw that predisposes it to accelerated wear
- assuming the use factors of the vehicle/equipment are typical of the design intent and not abusive or akin to neglect
- considering the vehicle/equipment is not modified in a manner which would escalate wear (heavy mechanical or electronic perf mods, etc)
- using a grade of oil that is generally acceptable by the OEM for the application

UOA wear data analysis has shown that a move from one grade to another (either up or down) does not significantly shift wear in a matter that we'd be able to discern. Engines that are spec'd for a 30 grade can safely go up to a 40 grade or down to a 20 grade and not see any major concerns.

There are some engines I've seen in particular that truly could not care less about what grade of oil is in the crankcase, as long as it's a proper spec for the application; their wear rates don't seem to budge much no matter what grade you use (Ford 2v mod motors; GM 6.6L Dmax, etc). Other engines are somewhat finicky, or have a design issue that causes concern (Chrysler 5.7L Hemi engines and their propensity to eat cams; oil isn't the cause, but it also can't seem to stop the problem).

It's important to use a lube spec'd for the type equipment. Don't use a GL-5 gear oil in your engine, for example. Make sure you use the right category of lube hierarchy (don't use a CF where CJ-4 is called for; don't use SG where SN is called for).

The caution would come from when you might alter one or more of the boundary conditions I set forth. I've seen data that indicates some things could go awry, especially if you combine any of those categories. Example: you have a "tuned" engine, use it for racing, and don't consider the elevated operating temps. Or, you live in a super-cold area where it's routine to see -15F starting temps, but use a 15w-40 diesel lube in your engine spec'd for 0w-20.

The data I've processed shows that "generally" moving up/down one grade isn't a big deal at all. It typically won't help or hinder the wear rates, IF the other conditions are not undesirable.

To the OPs question ... going from a 0w-20 to a 15w-50 MIGHT have an effect, or may not, depending upon those other conditions I speak of. He did put a boundary on the ambient temps, but didn't address the specific engine series, the expected use factors, the lube spec, etc.
The only thing I noticed going from a 10w-30 to 10w-40 (same Havoline oil brand and series) was a slight increase in operating oil temp. I attribute this to increased hydrodynamic friction from the higher viscosity, and haven't seen any increased wear from it.

One could argue that more viscous oil will trap air easier (or at least harder time letting it escape) and thus be more at risk of aeration. Higher viscosity would also increase oil pressure which can also increase aeration. You could also argue that the higher viscosity will mean more filter bypass events though unlikely at operating temp.

Something else I just thought of is some hydraulic lifters are sensitive to viscosity. Morel lifters come to mind with this. Higher viscosity makes them difficult to bleed down which can cause "over pumping" situations with ticks or could potentially starve the top end of the valvetrain (pushrods, rockers, and valves in OHV V engines). I see guys taking a fresh engine build, pouring 20w-50 in it with <70*F ambient temp, priming the system with a drill on the pump drive, and come in forums and social media asking why they aren't seeing oil at the rockers. Put something in it that's not 800 cSt and you might be able to prime it easier.
I once ran 15W50 in my old 97 Honda Civic. It speced 5W30. I remember it felt sluggish, lol.
I once tried M1 15-50 in my 1990 Ford Tempo. It was very sluggish compared to the M1 10-30 I was using. I left it in for about one week and drained it out. Also heavy oil reaches the top end slower in very cold temps causing top end wear.
I used 20w50 in a Avalon because the engine had low tension rings and would smoke when taking off at a light. I used the thick oil for years and the car ran fine when I sold it.
But does running thicker oil cause (or expel) more heat?

Basic thermal dynamics will have a thicker fluid take longer to get to temp and then at temp, take longer to cool down to ambient.

Your vehicle and oil cooling engine design is going to have more say as to how it regulates your oil temperature.
I dont need to think about my answer one second. I have been using M1 15-50 in everything with an engine since the dinosaurs were filling the bottles themselves. Multiple vehicles over 200k and one over 300k miles. Near impossible to wear out an engine using lubricant unless you really trying. Would the 10-40 or 5-30 done the same? maybe but the 15-50 has the larger anti wear add pack and sticking to it until they stop making it.
Specific heat increases with both increases in temperature and increases in viscosity. You want the oil to be hot. Heat drives reactivity which is going to make the oil work better for you overall. The 1 cSt lower viscosity from your oil running 10*F hotter in the summer isn't going to make a lick of difference in wear. Keep in mind these oils are put through Sequence III-H testing for 90 hours at 3900 rpm and 305*F oil temp against a pretty stout load. Your commuter car putting down the highway with 230*F oil temp isn't taxing that oil at all.
I dont need to think about my answer one second. I have been using M1 15-50 in everything with an engine since the dinosaurs were filling the bottles themselves. Multiple vehicles over 200k and one over 300k miles. Near impossible to wear out an engine using lubricant unless you really trying. Would the 10-40 or 5-30 done the same? maybe but the 15-50 has the larger anti wear add pack and sticking to it until they stop making it.
M1 15W-50 is a very good oil. It was my go-to oil in my 300ZX for a long time. I did a freezer test with it one time (my gf totally freaked when she opened the freezer and saw a jug of oil in there haha!!) and it sloshed around in the jug just like at room temperature.
Thin is in folks let’s drain them heavy ass oils out before summer hits . Let the 20wt flow like cafe wants you to . You may get a little more wear but it won’t stop your motor to getting to 300K 🙃