Thanks Shipkiller for your explanation, it's very accurate. I didn't know Mobil 1 switched back to Group III. However the information available on this subject are all unofficial as of course ExxonMobil keeps them private.
My original comment wanted to be a lot more practical. For example, there is no evidence that a Group V is better than a Group IV which is better than a Group III. Also evidences about synthetic being better than mineral are not so clear. There was a road test some time ago done by
Consumer Report that showed no strong evidences that 1) synthetic is better than mineral 2) expensive synthetic (i.e. Group IV or V) are better 3) extend oil change increase engine wear. Test was done on NYC cabs... There sure is a lot of “literature” and anecdotal story that say otherwise, but I'm just very skeptical of everything that shows no numbers or too many...
This said ,I personally stick to oil that are ACEA A3 and BMW LL-01 (so, fo me, it's either BMW High Performance Synthetic 5W-30 or Mobil 1 0W-40): this is what BMW engineers recommend so I don't honestly see why I should go with something else often advertised as "better" by marketing folks with little to no technical knowledge.
If you put in your car a oil that satisfy both ACEA A3 and BMW LL-01 standards and you get oil related problem the warranty will cover it regardless if it's BMW or Mobil 1. If the oil does not satisfy these requirements, they can give you trouble and say it's your fault cuz you didn't use the right oil. I don't know much about RoyalPurple or other “fancy oils”, so I can't really add to it: a thing I heard and I would be careful about is that “racing oils ” tend to be thinner than regular oil. They sacrifice engine protection for increase HP (most racing engines don’t need to last that long and they are tear down after each race and often rebuilt).
Btw, all of this does not apply to the ///M cars: they require Castrol TWS 10W-60. I believe BMW imports it from Germany and if they go through all that, I'm sure there is reason.
