Odorless mineral spirits, does a good job of cleaning and doesn't stink up the shop too badly. I clean it out 3-4 times per year and take the dirty solvent to the local repair shop where they add it to their waste oil tank and burn it in the shop heater all winter. I'm tempted to try a water base cleaner but I like the protection I get from the mineral spirits, leaves a very slight film of oil on the parts so they don't rust.
Mark
Mark