To really get it all, you're going to have to get the foam clean or it will eventually do two things. It will keep oiling the passenger's butt.....(hmmm) And, it will really start to smell, as it's organic and will decompose/rot/decay, particularly in the hot confines of a closed car in the summer.
If you can remove the seat bottom, you'll have to treat it with something to drive out the oil. Then you'll have to rinse, rinse, and rinse some more. It might never come out completely, or you may do more damage to the foam in the long term, with the cleaner you use. The cover should be the least of your worries as it will be the easiest to get clean. You might even have to buy replacement foam/seat bottom, whole seat from Nissan to completely rid yourself of the problem.
If you know of any car detailers in your area, you might run it by them. They may have some tricks up the sleeve. Or maybe check with Nissan?
If that gets you nowhere, try this guy.
I hear he's pretty good.