[Lemming]

Both Simple Green and ZEP can really mess up the finish of alum if left on too long, they are also not that great on grease (although excellent with oil). Gunk engine degreaser works well.

I just bought 5 gallons of Oil Eater cleaner and degreaser to try in my parts washer. I've not used it yet, but it is water based, environmentally friendly and supposed to be easy on alum and painted surfaces.