Just for the record, Mobil1 comes in 0W-40, 0W-30, 5W-30, 10W-30 and 15W-50. Ford Motorsports told me to use 5W-30 in my crate 351, which I use year-round. My pressure is never below 55PSI on the road.
Paul is right on the money when he says use the lightest weight
oil which will give you good
oil pressure. Viscosity which causes the relief valve to open will reduce flow to the places it need to go. Keep in mind that most of our cars have an
oil cooler which reduces the need for heavier weight oils.
Bob