I'm just about through my first tank of gas on my S2000. That's the tank the dealer put in it. I figure there is atleast a 50% chance that the dealer put crappy 87 octane in the car either out of saving a buck, because that's what they have on hand, or because they just didn't realize that the S2000 should run 91.

On my WRX, once the ECU pulls timing due to knock events, it's very stingy about giving the timing back.

So if I have been running 87, I figure the ECU may have retarded timing to compensate. If this is the case, will the ECU add more timing back in once I'm running 91 octane, or should I just go ahead and reset the ECU?

 

you can just put premium in. the ECU only retards the timing when it actually detects knock. since it will not be detecting knock, it won't pull the timing.

that said, keep in mind even with 93 octane, your ECU will pull timing slightly. the knock sensors are sensitive almost to the point of paranoia.