My old but not-so used Sawzall has started to run very hot in the gearbox. (The motor smells fine and is not hot). I thought that maybe oil had slowly been lost over the years or it had deteriorated. When I removed the gearbox (front) end from the motor (back end) I saw oil as thick as ketchup. It hardly flows at all. Is this normal? Does anyone know what weight oil I might replace it with? Thanks!