I work at a McDonalds, and trust me - I know about grease stains. Once, I dropped something (only technically my fault) in one of the fryer vats and was splashed with 350F vegetable oil. It got all over my uniform shirt (and burned the underside of my forearm a little, but I didn't whine or bitch about it, because it was partially my fault...). When I got home, there was a dark area all over the front of it. After letting it sit for like three days (it's a McDonalds shirt, who the fuck cares
) I decided to finally wash it. All I did was put in the normal clothes-washing detergent (Tide with Bleach was what we had at the time) and squirted a bit of [manual] dish-washing liquid (I think we had Palmolive at the time, but I figure anything decent would do. Maybe look for something that says it's good with grease?) into the wash along with the detergent. When I took my shirt out, it was as good as new.
Trying to think of this somewhat scientifically, I think that grease/oils are non-polar (for an example, water is polar, alcohol is polar - look it up on google), so any sort of non-polar solvent (benzene, ether, chloroform, and I think [because it contains benzene and some other things] gasoline. So, going scientifically, I would think that soaking the shirt in gasoline (or, if you can get ahold of it, benzene of chloroform would probably work better, and be less flammable) then **thoroughly rinsing it with water** (I wouldn't just throw it in the wash. I doubt there would be any sparks in your washing machine, but play it safe!), after the rinse, see if the stain is still there - if it is, try either a different non-polar solvent, or just throw it in the wash and see what happens. If the stain is still there, don't dry the shirt yet - drying it often makes the stain 'set' and even if you can get the oils out of it, it will have already damaged the material. I'm sure you'll figure out something. Just keep in mind that oils are fats and the whole idea is that non-polar means lipisoluable (which means fat-soluable, ie... dissolves fat - oils are fats, so it should dissolve oils).
By all means though - test a little bit of your solvent on an area of the shirt which isn't very visible (ie... if you normally tuck it into your pants, try somewhere near the bottom, otherwise maybe in the armpit or something), just to be certain that it won't stain it yellow or something. Given a white shirt, I would think that it would be safe, but better safe than sorry.