Ford Repair: Gas, octane gas, carbon deposits

Question
My mom has a '92 Taurus. I've been putting some of the higher octane gas in it, but since it's getting so expensive, I want to start putting the cheapest one in it. What if some of the higher octane is still in the tank, and I put the cheap stuff in it. Is that going to create a problem?

Answer
Robert-

  No...no problem at all.  The lower octane gas will just mix with the premium and it will create sort of a medium octane. Besides, in a Taurus I'd recommend using 87 octane or 89 at the most.  That engine isn't designed for 91 octane (or whatever premium is in your area) so the result is small amounts of unburned fuel that result in carbon deposits in the engine.  Hope this helps.

Steve