I've been leery on RP for certain uses. Like I notice they say 20w50 for gasoline engines. Fine if you were to use it on a liquid refrigerant cooled engine, but from what people told me that it can't stand up to the heat form air cooled or diesel engines, and there is specific oil for diesel applications that also are backward compatible on air cooled gas engines. I think it's the PPM of the Molybdenum within the oil, and diesel oils have a much higher count of Moly. All about the lubrication and stresses.
I use diesel engine oil in mine in 15w40 grade.