Don't use anything too harsh that may discolor the fabric, test the product on an inconspicuous area to see if it lifts the color on a white towel. If you can clean the fuel oil off your hands using warm water and dish detergent it'll work on the upholstery. The key here is to use an upholstery or carpet cleaning machine that enables you to inject the warm water into the fabric and at the same time vacuum extract it removing the water/detergent and oil residue with it. Be advised you will probably have to clean the entire seat cushion since you will be cleaning the aged soil as well and you don't want to end up with a bright clean spot in the middle.
If a large quantity has poured onto the seat and has absorbed deep into the cushion and padding beneath, surface cleaning will only enable the top being cleaned and the reside will continue to work up in time. If this is the case you may have to remove the entire seat, detach the fabric and replace the foam cushion you can buy at most upholstery shops. I believe, however, most auto upholstery has a plastic liner under it to prevent leaks from reaching the foam cushion, if this the case a simple extraction as above should clean it up nicely.
Steve