I have a 02 745Li. Aside from BMW oil, BMW has approved Castrol Syntec 5W-30 to be used. Have been using it with no problems since 02
Just an FYI. From an engineerinng stand point of view. Remember, these engines are overhead cam. The engineers has formulated the
correct viscosity in light of the fact the oil must get to the top of the motor as quick as possible. This is especially important in getting the lubrication
up to where the cams are. If the oil is too thick, it takes too long to be pumped up to the cams(very oil hungry). My engineering instructors used this
analogy....The oil must be pushed up to the overhead cams. This 10W or 5W is like the difference of pushing a 5 pound weight over your head vs a 10
pound weight over your head. And with overhead cams, you don't want to go too thick. Its 5W for that reason. Too thick and it will take too long to
lube the upper end of the motor. The days of overhead valve motors are coming to an end.