Z28

Hello, first time around C&G! I have a question about the Ocatane rating on my 1996 Z28 (roughly 60k miles, and stock). This car is NOT my daily driver, and I only drive it when the weather's good...so roughly 3-4 months out of the whole year. Now the problem. Since I dont drive my Z28 all that much, I figured I would let a family member take it to college for the spring-summer semesters. Well, I got the car back about two-three weeks ago, and really did'nt get the chance to drive it until last Saturday...thats when I noticed the engine was pinging like crazy at idle. My first thought was that stupid b* used Diesel or something, no she used E-85 whenever gas prices spiked above $2 (so yeah, my car's been running on that crap for atleast two months). Since Saturday night, I have been using 93...and does'nt seem like it did any good. I took it to the GM dealer today and they said they would do a full inspection but that would cost me lots of money I dont want to spend :( I am not a car nut, I can change the oil, thats about it. Do you guys have any tips or should I just shell out the cash and get it checked? Thanks for any advice.