I presume you're referring to the relatively lower octane rating of premium fuel in California versus that available in the Pittsburgh area? But I follow the gist of your question.
I know the Owner's Manual of my X3 recommends premium unleaded (91+ octane), and would presume the Manual for the M5 recommends the same; that would be typical of high-output engines with high compression ratios. I don't think (but admit I'm drawing on sketch memories) that the octane rating of premium unleaded is below 91 anywhere in the nation, so as long as the previous owner(s) always used premium unleaded, there shouldn't be any problem whatsoever purchasing a California-based vehicle.
That being said, even if the previous owner(s) used regular unleaded (usually 87 octane) in the vehicle, this is generally permissible, per the Owner's Manual, with some caveats about reduced performance. The computer (via knock sensors) will try to compensate for the lower octane rating by altering the timing to the extent that it can to avoid detonation. So long as the previous owner(s) didn't push the engine hard enough on regular unleaded to encounter detonation on a frequent basis, again there shouldn't be any mechanical problems with the vehicle. The way octane plays into the long-term effects on the engine is somewhat complicated, and dependent on the habits of the driver and conditions encountered (low vs. high altitude; very cold or very hot temperatures; moderate or wide-open throttle; etc.). Unless the paperwork history of the vehicle includes gasoline receipts, there's probably no way to verify what type of fuel was routinely used in the vehicle, and you'll have to determine for yourself whether to accept someone at their word if they tell you they always used premium unleaded.
I think in general you shouldn't have any fuel-related issues with purchasing a California-based vehicle; at least I wouldn't, given a thorough test-drive, and pre-purchase inspection. But ultimately the decision will be yours, and if you're not suitably comfortable with the unknowns, you might look at alternatives.
X3 w/6-speed & Sport Package:
A sports sedan disguised as an SUV...