What would be a concern with low RPMs?
Is high HTHS still important for a normal vehicle?
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Oil film thickness (MOFT) between parts is a function of the speed between moving parts. Journal bearings build MOFT with speed ... that's a characteristic of journal bearings.
Beside that, I always do my experiments on engines under warranty.
I put an HDEO SAE 50 in a one year old C-Class V6 Benz just to prove a point.
Might as well blow it up while it's on warranty.
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Well, lugging is hard on the rod bearings, more of an issue with turbo engines these days, but low rpm means higher loads on the bearings with less oil supply and less hydrodynamic pressure which increases with rpm. Probably subaru was pretty conservative with bearing sizes and loads and transmission tuning and engine tuning, but I would rather err on caution with something like rod bearings.
In my mind, and I'm no expert for sure here, when I hear "HTHS" I think high RPM/high heat...not low RPM cruising.
Regardless of the HTHS, the oil film thickness will be lower when the engine RPM is lower. That's how the mechanics work with an oil film between moving parts. The film thickness (MOFT) is a function of the relative speed between two moving parts with an oil film between them. With everything held constant, and oil with a higher HTHS will give more film thickness, which is the first line of defense between parts rubbing on each other and wearing.
With journal bearings, there is also a load factor involved when the engine is under high RPM and high loads. WOT high loads can reduce the MOFT in the bearing at certain rod positions due to extreme forces on the rods due combustion and g-forces due to high RPM. But even under those conditions an oil with a higher HTHS will give better protection from bearing to journal contact and wear.
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Who knows exactly what's "high enough"? HTHS headroom gives more protection when you don't know exactly where the MOFT is running at under all engine use conditions.
Yeah, it seems the engineers on the first gen ecodiesel in the Ram thought that too! 5W30 and extreme low rpm high load for mileage tuning killed lots of them. Mazdaspeed 3's engines could destroy themselves quite quickly too if you got into all the boost at low rpms to often.
I guess the high rpm and high mechanical loading failure mode, must be because of cavitation or oil foaming or something to kill the plain bearings in bottom ends of motors? You wouldn't think the mechanical loads would surpass the oil film load capacity as you would think they both increase exponentially with rpm? But I'm no mechanical engineer or tribologist.
I read the manual, and/or find out the oil spec minimum requirements for the engine in question. If the car manufacturer says HTHS X.X or better that would be an indication to me that this HTHS will be sufficient for the vehicle in question under normal conditions. If I were to track the vehicle or tow a boat I may want to go higher. If the vehicle in question is a high-end sports car I would not presume having to go to oil with a higher HTHS. For example, I know my Mini requires HTHS ≥3.5 because that's part of LL-01.
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Two things are going on at the same time in rod bearings. As the RPM increases the bearing tries to center itself which increases the MOFT. But also as RPM and cylinder loads increase, there is more load going down through the rod into the bearing which want's to decrease the MOFT. As mentioned above, even under those conditions an oil with higher viscosity/HTHS will result in higher MOFT.
I think broad statements are now hard to make. New cars with wider, coated bearings may be just fine with low HTHS. Some designs just won't be fine, ever. For example, BMW S65 and S85 still eat rod bearings for lunch despite running 10W-60 with HTHS of 5.2. Oddly enough, many people run Mobil 1 0W-40 in those engines and while it does not seem to help, it doesn't seem to hurt either.
Manuals (in the USA at least) are specifying oil based on an absolute minimum to meet CAFE while at the same time not make the engine fail. But that doesn't mean that the CAFE driven low oil viscosity called out in the OM is giving the best protection from wear. There is a trade off between increased fuel mileage and increased wear as the oil gets thinner and thinner.
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4wd
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At that point - why not 5W30 ?
I sleep very well at night - or anytime.
Engines specifying 0W-16 or 0W-8 definately have some special design features to run that thin of oil. Those oil viscosities have their won ILSAC GF-6 designation and API symbol to warn people not to run those oils in an engine specifying xW-20 or above.
You should with a specified oil of 3.5 HTHS or more like your manual says. You willing to try some 2.4-2.6 HTHS?
The official minimum requirement is my personal threshold.You should with a specified oil of 3.5 HTHS or more like your manual says. You willing to try some 2.4-2.6 HTHS?
But would you think there would still be the same level of engine protection if you ran a 0W-20 with a HTHS of 2.5 cP in that engine? That's what I'm asking. Why do you think your OM calls out oils with 3.5 HTSH or more? Could it be that the engineers know it's going to give more engine protection (and care less about CAFE) than an oil with HTHS of 2.5 cP? I'd say the answer is yes.
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