When it comes to gas, expensive does NOT equal better! Now, the engine may require higher octane gas, but the concept of calling higher octane 'premium' is purely a marketing idea designed to trick people into thinking they are putting 'better' gas into their engine. Higher octane just means it burns at a higher temperature, not that it makes more power, or is cleaner, or better, or anything. My friend's race team runs 87 octane because thats the way they tune their engines, low compression and EGTs w/ high volume. Your argument that it is an expensive car and hence needs expensive gas makes no sense. Choose the octane that is best for the engine's design, whether that is cheap or expensive, that will be the best gas for the car.

That said, the engine may need the higher octane based on higher compression. I don't know exactly where the chip is on the engine, so I can't advise there. Can't you ask the PO? Or are they long gone? If you can't find out, you can always run 91, which is a good middle of the road octane.

Peace,
Sam

__________________
"That f***in' biodiesel is makin' me hungry."

1982 300TD Astral Silver w/ 250k (BIO BNZ)
2001 Aprilia SR50 Corsa Red w/ 5.5k (>100 MPG)