OCI requirement for LL approvals is based on EU market, not the American market.
There are specificities of NA market and utilization of vehicles. Initial 15k OCI was based a lot on EU market, not taking into consideration high sulfur levels here that were present until 01/2017. That is what prompted a reduction in OCI.
Yes, I have seen the internals of BMW engines, and not just BMW's I had. It is extremely rare for BMW's to develop any kind of sludge visible once VC is taken off. If it is present, that means not just that OCI was long, but exploitation of vehicles is a problem (short trips), and other neglect.
N52 engine on NA market in E90 did not have any kind of oil cooler, heat exchanger, or radiator type. Regardless that it was port injected engine, it was very hard on oil due to very high oil temperatures. Yet, I have seen N51/52 engines racking up 250k and people still tracking them in 128 or 328.
And when discussing oils, I do take into consideration OCI of any specification. Still, it does not change the fact that you are using the wrong oil and your UOA shows that.
I have found the opposite to be true. BMW engines, regardless of type, year etc. seem to sludge up more than others I have worked on and own. I am not talking about just my experience either, I know retired techs who have seen this also, plus the internet shows multiple examples. The highly evolved service indicator system should be able to compensate for abusive scenarios, and pull back OCI accordingly.
My position is this:
BMW used the sulphur excuse for their V8 engine problems in the 90s, which may have actually have been the case. I don't buy it for this.
Somewhere in that firm they have decided on a philosophy of long drains and lifetime fills.
Lifetime fills in GM, ZF, and Aisin built transmissions that other firms use with more reasonable (realistic) intervals.
I love BMW and think their cars are excellent. But, they do make mistakes, which somehow, they never admit to.
For the enthusiast, BMW recommendations should be considered to be for a different purpose than solely vehicle longevity.
I mean fluid type as well as interval in the above statement.
When people only recommend going with a fluid that meets the cert, I think that is certainly a safe bet, but is also not the end of the discussion, especially where BMW is concerned.
And for myself, for disclosure, I have a bias toward experiment. I usually try the cert, and then begin to use something else, maybe. Sometimes its not any radical shift, for engine or transmission, sometimes it somewhat is, like 20w50 in a 5w20 spec engine. (Rotary).