Gasoline and brake clean will do the same thing, gasoline will be cheaper. Fill it up run it a while with the drill or just slosh it around if out of the truck. Do this a couple of time and it will be clean with no harm done. Diesel and kerosine will also work, just not quite as aggressively. Once the gas drys out, fill with gear oil and slosh it around to coat the insides. My lawn mower has no drain plug, so I have to pull the engine off and turn up side down for draining, I then fill with gas to clean out the bottom. It hasn't hurt the gaskets yet! I work with a group of guys at work that love to use brake clean, I find its very hard on skin, and lungs, plus if the vapor get burned by a local furnace or fire, the results are very toxic. It does an excellent job of cleaning, just use it with caution.
