try a citrus based hand cleaner and you could use a spray bottle to add water to it if it needs any agitating to help dissolve the stain. then use a wet/dry shopvac to suck it out once it has had a few minutes to soak in.
if it leaves a clean spot you'll end up having to do all of the seat to get it to look good.
i turned my shopvac to blow after i got the stain out to dry the seat faster.
a citrus based engine cleaner??? not sure if it would be too powerful or not, just another thought....
keep smilin.........t