Quote:
Originally Posted by MPR
I thought about switching to a 30 weight just because more viscosity sounded better but decided to stick with a LL17FE oil, specifically LiquiMoly TopTec 6600 0w20. I trust who engineered the engine more than what I read in a forum or my caveman level of oil knowledge.
The clearances on bearing surfaces and oil passages are designed for a specific flow and thus a specific thickness of the oil (at temperature). If your oil is too think it won’t flow good enough and the bearings will be starved.
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Not a scientific study but I switched my 996 Turbo from 0w-40 oil to 5w-50 oil. As best I could tell the hot idle oil pressure remained unchanged. What 5w-50 brought with it was better HTHS (high temperature/high stress) number compared to 0w-40 oil.
Oh, and the W viscosity was 5 rather than 0. Porsche was ok with this as long as the car was not used in cold temperature. The threshold was -17F (-25C).
A too high a viscosity of oil can be as bad as too low. As you touched upon clearances are important. Main and rod bearing clearances can be as low as a 0.001" or so. Too much viscosity can impede oil flow into and possibly out of the bearings.
Hydraulic lifter clearances are even "tighter", precise sliding fits. (A Porsche hydraulic lifter rinsed with brake cleaner (no residue) still manifested resistant to compression due to the air being "trapped" in the lifter. The fit was that close.)
There is oil flow concern but also heat. A more viscous oil can run hotter in the bearings and hotter enough to possible risk oil break down.
Found these at the Castrol web site. Castrol makes/bottles the BMW oil is my information. I'm not suggesting one use Castrol oil in place of BMW oil I'm only providing this info for comparison sake. I believe the numbers would be the same if one had the same product data sheets of BMW oil. But no guarantees. Wonder if I could get product data sheets for BMW oil as the dealer?
The takeaways I note are the viscosity at 100C is 8.18 (0w-20) vs. 12.3 (0w-30) and the viscosity index of 168 (0w-20) vs. 205 (0w-30).
But as I mentioned above and as you touched upon "more viscosity" might not be better...
However, at least in the case of my 2024 230i xDrive BMW (at least in the owners manual) allows for the use of 0w-20 or 0w-30 oil. I have not confirmed 0w-30 oil is ok to use with my BMW dealer.