The owner's manual calls for 91, but I'd be stunned if using 90 would make any difference whatsoever. Besides, it's my understanding the rating on the pump is a "minimum" rating....it may actually be higher.

ETOH only has about 65% of the energy as pure, so a 10% blend has about 96.5% as much energy as pure...no matter what the octane rating is......93 pure has no more energy in a gallon than 87 pure would have.

Driven identically....the pure should get you almost 1 more mpg in an XF.