Engines need oil flow and pressure is a characteristic of flow and viscosity. If you increase your viscosity unnecessarily, you could end up causing more flow to recirculate back to the sump through the relief valve. I would use a 10W-30 heavy duty engine oil (I use Petro Canada Duron SHP 10W-30) in my engine with a dual rating (ie, API CK-4/SN) that also meets Ford F1WSS‐M2C171‐F1 (for more ZDDP).

Your donor truck's owner's manual would likely have called for 40-grade oils in summer temperatures. You can easily use any 15W-40 heavy duty engine oil and this grade's higher HTHS viscosity will give your bearing more protection. See Corvair Oil Guide and Engine Wear.