Short answer is stick with premium.

I think with a 10:1 compression ratio, and a DOHC head, that 87 will result in the the engine retarding the timing to prevent detonation, which in turn will prevent damage to the engine but will result in a loss of fuel economy due to the fact that less energy will be harnessed from the fuel becuase the later timing will result in a loss of efficiency. So, you may be able to get away with 87 without engine damage, but MPG will drop as a result with no net savings in the end.

Nice signature. Unlike an aircraft piston engine, which is purely mechanical in nature, the M104 can detect detonation and prevent it but sacrifice efficiency in the process. Your standard lycoming or continental will just get blown apart if its bad enough. Funny how a an engine with no higher than a 8.5:1 compression ratio and single overhead valve needs 100 octane fuel to run. Most of the older engines that can get STCs for 87 octane car gas have compression ratios lower than 7:1.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record