getting the engine tuned up and cleaned out would be a good idea. unless it says so in the manual, your car shouldn't need premium and that it does is a sign something's wrong. probably caboned up combustion chambers. that'll get you a few mpg and allow you to run regular. do the ol' seafoam treatment to help with that.
full tune up and filter change is always a good idea unless you know when they were last done and that they're good. (however, an ignition problem will NOT be fixed by running higher octane so that's not the primary problem)
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"