I dunno how good it would be for the cases, but with bikes, we take and put in some lacquer thinner and about a half quart of oil, run it for about 30 sec or so, drain, and refill with fresh oil.  It doesn't hurt the wet clutch or the trans gears, but I don't know about how the Toy case would react to it.  You might try WD 40 (it's hygroscopic).