For grease stains I just use dish washing soap. Smear some one the stain and let it sit for a few minutes. Wash out as much as you can by hand and then wash how you normally would. You might have to do this a few time depending on how tough the stain is but works like a charm.

Hey Matt,My old shell got some serious grease stains that got washed in a couple of times, maybe 3 or 4. I stressed but then my wife used shout advanced gel. The thing you have to do is apply it very liberally and really grind it in. The apply it 3-5 more times the same way. Then you want to wash it the next morning. It got the grease right out of my jacket.There is some stuff that will get the grease out but it will also damage the integrity of the material. Hopefully that will help.----Thanks Steve for your answer. Just want everyone to know I did what Steve said and it did wonders!! Everything came off. Here's the step-by-step guide.-Poured water on the grease/stain spots for few minutes and rubbed those spots with my fingers under running water so the spot areas got wet.-Applied Shout Advanced Gel. Like Steve said, I ground the stuff on the jacket quite hard and quite a bit. I had doubts how Gore-tex Pro Shell would take the punishment and believe me it came out like new.-Waited for half an hour and applied Shout again and then waited 30 min and repeat (I applied Shout total of 5 times, always waited 30min before repeating the process).-Let jacket be over night (this is the most important part).-Next morning washed it with Granger's G-wash (warm water). I'm sure Nikwax will work here too.-Tumble dried with delicate setting.Now jacket looks like spanking new. Remember that before I did what Steve suggested I washed this jacket twice with Granger's G-wash and once tried Shout without any success. So the key here is to apply Shout multiple times and letting Shout sit on your jacket over night.