Towne BMW here in Buffalo recommends the mid grade, or 89 octane, to all their customers except the M cars and S Minis. We have driven both our cars a total of 70,000 miles on 89, and there has been no problem at all. Even Towne's top tech, who is an award winner, uses 89 in his own X, which is a 4.4 . . .Notwithstanding BMW's official recommendation, 89 seems to work well, and there is little, if any, power loss. Certainly not enough for the average driver to notice it.

In the final analysis, however, it's up to you. Good luck.

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Sageriders
2011 328, Water [blue], Saddle
2006 X5 4.4i, Sterling Grey/Truffle, Premium, Sport, Cold, Rear, Adj. Ride Ht.
2005 X5 3.0, Sterling Grey/Black, Sport(Totalled!)
2002 330i, Grey-Green Met/Truffle Br. [Sold]