Yeah, the Mobil guys did admit that synthetic, thanks to its slipperiness (technical term
) would leak past some gaskets where conventional didn't.
TO answer Bill B's musings - The data I saw was presented by Mobil engineers, who we must acknowledge up front are likely biased. But since this was a technical society presentation, from engineers to engineers, I would hope the bias wouldn't skew things too much. That said, they emphasized that since additives are pretty much proprietary and govern oil's long-term stability, they claimed Mobil-1 as superior to most other synthetics. However the data they showed looked to me like there wasn't a lot of difference between synthetics, at least nothing near the difference between synthetic and conventional. Regarding the individual characteristics, I think they addressed that, but I don't recall the outcome (I was and am using Mobil-1 anyway, so once I had the smug assurance I was using a good product I didn't pay attention to such detail, sorry).
Regarding BMW-specific oil, I plead the 5th, as didn't previously know there was such a thing, so I am Sgt. Schultz on that topic.
The start-up issue is a big one. Yes, that crank is just sitting flat on that bearing when it isn't turning, but once it gets spinning it starts riding on that film of oil, not touching the bearing at all (or so goes the sutomotive engineering chapter on the subject). So while start-up wear is undeniable, I must point out that every time I've looked at engine bearings on used engines (I'm not a pro mechanic so the number of times I've done this is about 5 to 10 times), I've always seen oil coating 100% of the surface. So my instinct tells me you get "some" benefit at start-up from this entrapped oil, the Q is how much.
I guess I should have pleaded the 5th before I started typing this. Sorry so wordy...