seems like there is a controversy about that from the little I read: water-based detergent solution or hydrocarbon? I plan to use it to clean bearings, Birfs and the like. Seems like Diesel would do. Cheap and not too flammable. But my cleaner is a big 20 gals and may take quite a bit for the pump to go -although I could put some bricks in there to raise the level. Plus would smell and all. And I would use this only infrequently so storage issues. What do you use and why?