No, no, you're getting mixed up.
Although thicker oil doesn't physically affect the clearance gaps, it very definitely means it won't flow through as quickly! Yes you'll have higher oil pressure when the engine's cold, but that's because it's harder & slower for the oil to go through all the bearings etc.
With tighter clearances, when the engine's cold (any engine), which means when it's fully cooled down, (not just when ambient temperatures are low), the oil (any oil) is thicker & takes longer to be pumped around the engine than when it's hot (warmed up engine).
When you do a stone cold start it takes
a second or 2 to build up pressure, which means (on any engine) on starting* you've got metal to metal contact briefly, which is worse on a cold start (engine cooled right off) as the oil takes longer to go around. Add to this the overly tight big end bearings clearances the S65 has (including the later alu/tin bearings to a
slightly lesser degree), it means the oil takes a bit longer still to go round, & you're compounding this by using a 20w-x oil instead of a 10w-x oil.
So even if you're coldest morning temp is ~20C, you're engine is definitely worse off with higher 'w' rated oil.
*btw, bearing manufacturers have had to make 'tougher' bearings for cars that have auto stop-start! I wonder if that's made the S65 bearing issue worse in later S65s??.....
Yes, doing a cold start in cold ambient temp's makes this much worse (you likely wouldn't have got away with it to date with freezing temp's!), but seeing as theirs fairly good evidence that the S65s tight big end bearing clearances are the major cause of premature wear or even failure, putting thicker (cold rating 'w') oil in your engine is making this more likely!
Honestly mate, I think you're really risking it putting a thicker 'w' oil in it! I wouldn't want to see you posting a thread further down the road saying 'rod bearings failed!'
As you say the 50/60 oil rating discussion is another story, but their is sound reasoning for going for a 5w-50 oil instead of 10w-60, and this is done primarily to get the thinner 'w' rated oil (not the hot 50 rating) to improve oil flow on (engine) cold starts. The trouble is all oils shear (wear) down (long before their service life in some cases), which means they become thinner at the top rating at least (and I believe the colder end becomes thicker too).
So e.g. you can see in many oil reports that 10w-60 shears down to a 50 grade oil after a few 1000 miles typically, and a 50 oil will shear down to a 40 grade oil, some oils will hold onto their fresh grades longer than others.
The other issue with using -50 oil is that it could potentially be too thin under arduous conditions, & when it's sheared down some. Although I haven't come across a failed engine due to that yet (early days for me reading about this though!).
The ideal oil would be a 5w-60, but that doesn't exist.
Anyway, in any climate, with the S65 engine, you really don't want to be going over a 10w- oil.
Read more about oil gradings
here
=> https://www.m3post.com/forums/showthread.php?t=1404625
