I run Diesel-rated 15W-40 HD oil in everything I own, including a couple of high-mileage engines that were specified for 5W-30 when new. The main reason for using water-thin oils is for an imaginary gain in gas mileage for some of the new engines that are subject to EPA regulations. The clearances on bearings, piston skirts, etc. are unbelievably close, like less than .001" on piston skirts and crankshaft bearings. What used to be acceptable clearance 50 years ago is "worn out" now.

Mobil 1 is OK, but if there's even a little bit of oil consumption or leakage between oil changes it's going to get pretty expensive. It's hard to beat a high detergent mineral oil with some ZDDP additives to reduce wear on the flat-tappet camshafts these old engines run. Newer oils run roller cams, and they don't need ZDDP in the oil.
Jerry

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