Spraying it out with carb or brake cleaner should work. If not, you could pour some Berryman B-12 or acetone into it and prop into various positions to let it soak.
I've heard of people using oven cleaner for stuff like this but I've never tried it.
Bead blasting would work if you're bound and determined to get it spotless.
Or, you could take it to a machine shop and let them put it in their parts cleaner.
HTH,
///Mike