But downgrading means you get less MPG. [/quote]
Are you sure about that, Ice? I know that overkill doesn't get better MPG. When they say minimum 91 and then use premium, that confuses me. The car will obviously run on 87 or 89 so 91 isn't likely the true minimum. So maybe the BMW conservative minimum is the one that will get the max MPG. But, if you are pretty sure, I will certainly take that into consideration.

 

But as I stated earlier on, downgrading also decreases performance right?

You use cheaper fuel, you get less engine responsiveness. If you spend so much money on a nice car it'd be quite sad to have a nice V8 only to restrict it to US91 or even US87 fuel.

If I had a V8, I'd definitely just use RON98 = US93. Even with my 530, that's the only fuel I use.

 

That's the one BMW recommends (over here in Germany).

 

I never tried it myself... So this a: NO!
I THINK it is like this! After reading on the internet and talking with friends about it I came to the conclusion that they are right. Because if you talk to 15 different people who are somehow BMW or oil company related, and 13 of them say it is like this, and the other two say they think it is like this but they are not sure... and nobody of them tells you that it's no problem at all... I stick with the highest available fuel - at least the 98 GER (93 US) octane.
In my case, I only use the 100 octane...

 

I was woundering what you guys would think if I ran a mix of Sonoco 110 and premium 93 50/50. Thats the way I run in My Motocross Bikes and it works great.