My parts washer has the solvent type mineral spirits. It is made specifically for parts washers. A local fuel distrubitor sells it, and it is despensed from a standard type gas pump, but it is only avaliable at the distribution store, not the normal fuel stores. Anyhow, it works good, but is really hard on the hands/skin so I have some nice chemical gloves to protect my hands. I have thought about going to a water based cleaner, but my parts washer in in an unheated garage, so freezing would be a problem.
A friend of mine who has a transmission shop uses a water based solvent (not sure which brand), but it worked really well and is easy on the hands. I think it comes in powder form in 20 pound buckets?