Let us know how it comes out.
20-50 is a really good oil for a well worn engine, it takes up a lot of room in the bearings, down side to it is it thick and takes more horse power to spin the pumps plus a lot longer to warm up. I have seen distributor gears worn 2/3s of the way away from thick oil, it just puts to much of a load on the gears trying to spin the pumps. You are right to run thick oil in high heat like you have, but modern oils are so much better then they were years ago, I think you can get away with thinner oil and still have the proper protection.
Joe
