I run 89 day to day , but if im driving hard or for a trip up thru the mountins i run 91 . My friend at Ford , he did the testing and crap on the SVT in Detroit . He said its just fine but advised me agains 87.
You really should stick with premium fuel w/no less than 91 octane. Our engines are high-compression engines (10:1 IIRC) and anything lower in octane can cause detonation (especially under hard acceleration) which can cause severe damage. As stated above, better safe than sorry.
10.2:1 i will NEVER use anything lower than 93 i don't care how much gas prices rise. i get 30 mpg anyway. i'd rather have a good running engine than having one with problems just because i wanted to pay 10 cents a gallon less for gas.
i ran cheap gas for awhile and i noticed that my car was starting to run like crap so when i went to change the spark plugs i found that number 2 cylinder plug electrode was burnt gone!!! so in short dont use cheap gas... fortunitly it didnt hurt anything on my motor but it could have!!!