There may be discussions on the forums already, might be worth a search. What gas is recommended for your car? If your manual, gas-cap, or filler-area specify a particular octane, that's what you should use, at least. The more expensive, higher octane fuels might offer some slightly better mpg's, perhaps, but not substantial - maybe something like 1 mpg. Besides the higher octane, I think the premium fuels are supposed to have more in the way of an additive pkg. than their less-expensive counterparts. In the short run, that may not be significant for you; if you're planning on running your engine/car up to 200k+ miles or more, and you want to minimize deposit buildups in the engine, you may find premium gas helps keep engine internals a bit cleaner. YMMV & no guarantees, but if minimizing buildups is important enough to you, you may find that a reason to justify opting for something other than baseline. I don't have specific insight as to whether any particular fuel octane is a problem for M54's or not.
