My experience with this is from the practical side, and is specific to experiences I have had (and discussed with others) mixing race fuels and "pump" gas.

The pump gas that we normally used had MTBE or other oxygenating additives, so this might have led to it.

I have seen very unusual plug chops (run engine wide open, top speed, for 1 minute, turn off and coast to stop, pull spark plug and (hope) to determine jetting conditions) when using a mix of pump gas and race fuels such as Trick, Sunoco 108 unleaded race, and others (which I imagine are mostly the same stuff with different labels). We saw what appeared to be debris, which was actually congealed and burnt stuff(?) that was sticking to the spark plug (all over, but certainly not evenly distributed).

A rep from Sunoco claimed that certain pump gas additives caused the race fuel additives to fall out of solution. I haven't heard this exact explanation elsewhere...

As for mixing pump gas (91 with 89 or 87), I don't think there is any problem with this - most pump gas is the same with more or less additives depending upon which brand or which grade you end up with.

-DavidL
92 500sel
82 QPort
93 ZX11
86 535i