Use the lightest
oil you can while still maintaining the pressure you need! Heavier
oil may go from the pump to the
oil pan via the pressure relief valve. Asking 20W-50 paraffin to go through the filter, the yards of plumbing and the oil cooler at 40 degrees is asking a lot. If you can use 5W-30 and still get the pressure you need, use it. If engines also had flow gauges (in addition to pressure gauges) many drivers would use thinner oil. You can't go wrong with Mobil1.
Bob