I'll second LarryBible's post.

A few times I deliberately put Regular in my Audi (because it was cheaper) even though it has a Premium on the fuel door.

Nothing happened.

Well, performance did suffer a bit, probably a result of the knock sensor retarding the timing. I never heard any pinging, so the sensor was doing its job.

Go out and cruise around until you need another tank full and fill 'er up with Premium.

BCingU, Jim