My vote goes to mineral spirits. Here's a tip for those of you that live in colder climates. When cold weather gets here in Minnesota, I drain my parts tank into a 5 gal. bucket.... add about a quart of water to it and set it out side. The solvent will float on top of the water.... the crud will sink to the bottom of the bucket.... and the water will freeze. Now pour off your clean solvent back into your tank.

For you guys in a warmer climate. Take a 5 gal. metal bucket and install a valve about 3" up from the bottom. Fill the bucket with water stopping just short of the valve. Now drain your solvent into the bucket. Let it set for a day or so and open the valve to drain off the clean solvent.

I keep two batches of solvent on hand so I have a standby in the summer.