Given how this car bites back all the time, I've agonized over this decision. I've decided to go to Amsoil 0W40 European formula for the cooler weather and winter. I'll be changing the oil this weekend, but not changing the filters since they've only been in place for several hundred km. I'll be adding MoS2 again, because .... WWII fighter planes with no oil due to bullets in the pan, still flying.
Part of the impetus for this was discussions on this board and here on the M3 board.
. Apparently there is a lot of engine building / tearing down experience. One poster, Kawasaki, designs or builds Nascar engines. From all this reading everywhere, it seems that the bearing gap is not sufficient, based on industry standards, for the weight of the oil. JColley and B767capt (sorry if I've misspelled it from memory) and others have talked about this as well. I have to believe this experience and expertise but I'm not sure that this is the definitive cause of the bearing wear, although it may be a contributor.
My own intuitive idea, unbacked by any industry experience, is that there is wear from the 10W at startup, especially through small spaces; that overfilling of the oil system with oil changes (and you know this has happened in the past or that second drain plug would never have been added to the sump) leading to cavitation with the small air bubbles trapped in the heavier oil (and not being able to escape like from lighter oils), the driving at less than race track levels, and shorter trips that leads to fuel in the oil (and possible "scrubbing" of metal surfaces from this); all of this can contribute to bearing wear. I think that if the bearing gap were really TOO SMALl for the oil, that ALL E60 M5s would have had early bearing failure, no?
SUMMARY: Here are the assumptions behind my decision.
1. The bearing gap is smaller than it should be for the oil weight which could exacerbate wear from certain events;
2. The car is happiest when ridden like a *****; It needs to get to operating temperature for quite a while;
3. Most E60 M5s are driven at less than this level and this leads to fuel in the oil;
4. Any overfill of oil - which is easy in the E60 M5 - leads to minute air bubbles in the oil which do not clear like in lighter oils and cavitation is a source of wear;
5. Most engine wear occurs at cold start. This would be more so in colder temperatures. 10W is a heavy weight for cold weather.