Most any servo should work. The trick is to not use a stiff throttle pushrod wire, as it transmits the vibrations back to the servo then. But some servos have more fragile servo gears in them too. So the guys going with metal gear servos isn't a bad idea. But a standard Futaba S3003 or S3004 servo would work fine too. Also make sure the servo is not jamming up against the stops and WOT or low speed too. If the servo stalls out at the extreme ends for too long, it tends to burn out the motor controller inside the servo.