FSR is rather effective at cleaning stains. Practical sailor rated it right up there.
Good old Spray 9 was the number one if I remember correctly.
Bleach is a last resort, but works as well, you'll have to go with a stronger sealer when done (poly whatever) to seal out further stains, and seal in what ever oils and bonding is still left.
If all else fails maximum degreaser and a a good
paint job fixes the eyesore but not the underlying problems.