Main question is: Got a new S4 and it's rated for 91 octane. However, the local Costco, which I frequent, only has 93 (and 87) and it's the same price as 91 from other stations which are less convenient to visit. Is there ANY downside to 93 over 91? Mileage, power, damage, whatever?

Also, too late now, but I never got to test a Drive Select package because they're friggin rare around here (none within 3 hours' drive). Does anyone know if there is any real benefit to it, from a performance standpoint? (Not the sport differential--I did get that.)

 

93 should be just fine. Do not go with the 87. The ECM will compensate for whatever grade of gasoline you go with but you will get better fuel economy from the higher grade. The main reason higher octane is recommended has something to do with extending the life of turbos but I don't know what the reasoning is behind that.

Not familiar with the "Drive Select package" so I cant comment.