Sorry if this is in the wrong section, since it's not related to the s2000.

My girlfriend recently bought a base mini cooper whose previous owner used regular gas for over a year. She didn't know that minis required premium 91 grade gasoline until she read the manual. Is this a big problem? What problems can occur with the engine in the future?
I used to own a g35, my dad used to burrow the car and filled up my tank with regular 87 gas several times before i found out, and i didn't notice any difference in performance or even engine sound, and had no problems at all. But the mini ran on 89 for over a year..

 

No, the problem is she bought a used mini cooper. Good luck

 

Why is buying a used mini cooper a problem?

 

Why on earth does a 120hp economy car require premium fuel?

 

^ thats what i thought lol

 

all cars are able to run fine with non-premium gas. it just wont perform as factory standards.

 

I ran 89 in my WRX once because I was in the middle of nowhere and that's all the station had. I felt the difference immediately. It ran like crap.

 

who cares it still averages over 30 mpg. My sister's base cooper averages 31 mpg in NYC driving.

 

It depends on the car, the gas, and the conditions. Some cars will adjust timing to deal with the lower octane and you'll definitely notice the difference driving it.
I think one of my friend's G35 would throw an engine light, drive like shit and get like 10mpg during the winter if he put regular in it.

 

Very few cars "require" premium. As was mentioned the ECUs just turn down the power when you use regular. Actually most of the time they won't turn down the power but if the engine detects knock then power will be turned down. Off the top of my head the MS3 is the only car I can think of in recent times that requires vs requests premium.