The service manual for my neighbor's Cummins 450C engines specifies a special coolant that meets a GM spec. Does anyone know if we can simply use a 50/50 mix of Prestone or similar off-the-shelf product? The engines are no longer under warranty (I encouraged him to use the Cummins fluids in the past to cover the warranty period). The oil, coolant, etc. are very expensive, and he'd like to see if he can save a few bucks on the routine service.