From what I read on the pre '04 engines they are designed to run all the way down to 85 Octane without serious issues just a reduction in performance as has already been mentioned.

Ran my '01 on 87 every other tank and never had a problem with it, it passed all the maintenance checks fine up until it was lemoned at 30K miles for the fuel tank problem.

Most engines today are pretty sturdy under pressure, you may have some valve pitting due to some pre-ignition problems but after leaded engines became history engine technology has improved dramatically.

Take driving in Africa where fuel can be dubious in quality and yet you have a lot of older cars still running no problem.

In Europe they are adding cooking oil to their diesel to minimize the costs over here as the cost of cooking oil is so cheap and apparently as long as you do it in moderation there is very little impact to the engine.

BTW anyone have the Octane conversion for European to US, I believe 95 is the same as 91 and 98 is actually more like 93. They believe in more expensive cleaner burning fuels in Europe, methinks. My '05 4.4 runs fine on 95, no performance or mpg issues.

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Currently 2010 X6 35i, Premium, Technology, Sport.
2004 Lotus Esprit - Final Edition
Former '04 X5 4.4i, Highland Green, Nav, DSP, PDC, Heated FR & R Seats, Comfort Seats, Rear Load Floor, Premium & Sport Package, IceLink Plus, Bel RX65, Tow Hitch, Sirius Satellite Radio.
Former '03 X5 3.0, Nav, Premium, Rear climate. (Stolen)
Former '01 X5 3.0, Nav, Premium, Rear Climate (Lemoned)