Stoddard Solvent, which is kerosene based, works well. It doesn't attack the rubber or lift the paint. It can be run through a fine mesh filter and reused until it's too thick to strain. It doesn't evaporate as fast as thinner, so it will set and work on the really crusty stuff. You can also dip parts in it overnight without worrying about the paint and it won't fade the paint either.
This is the stuff you used to see in parts washers until Safety-Kleen scared everyone. Ask for it at the auto paint store or machine shop supply house.
By the way, Safety-Kleen solvent may save the planet, but it won't save your time cleaning a part.
