Damage would occur to the engine if the oil pressure is too low. If it is not, then follow the recommendations in your owner's manual. There is a chart in there that shows what viscocity of oil to use according to what the ambient air temperatures are. What kind of climate is the car exposed to?
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record