I would try to loosen as much as you could with just water pressure first. Let it sit for a bit to drip dry. Then the rest is up to you. It depends on how safe you want to be for yourself and/ or the environment.
By far the best thing to remove petroleum based soils is a solvent. If you are comfortable with this, a lot of shops use cheap brake cleaner; it has a jet spray that helps with agitation and the solvent works ok on the greasy parts. It dries really quickly too, so you won't have to rinse the parts down.
Another option is a heavier duty solvent that you would let sit. You could use a petroleum based, citrus based, or soy based and they come in liquid or foaming. Personally, I get great results from using a soy based foaming solvent; its "green" and the foam lets it cling to the parts and really do a great job.
If you don't want to use a solvent, you could use a butyl based cleaner or a caustic. Most caustics are a blend of NaOH and KOH and water, and can be diluted or used straight. I don't prefer these because of the dangers of skin or eye contact, but that's just me.
Butyl based cleaners work great (well, the good ones work great), but aren't environmentally friendly.
Then there are the "miracle cleaners", like Purple Power and Simple Green; eco-friendly, you can feed it to your kids, bathe in it... you get the picture. Personally, I can't stand simple green; I've never had it perform well on any tough degreasing job. I can't speak for PP. As far as I'm concerned, YMMV on any of these products.
Hope that helps.
** I should add that both solvents and butyl cleaners can be harmful to painted surfaces, and solvents can damage plastic if left on for an extended time.
