M1 synthetic is 15W-50 and 10W-40 last time I got some.
Simply stated, you want to run the least viscous (thinnest) oil that maintains pressure at all times. Hot idle is generally when pressure drops the most. If 10W-40 works well for you, I would not necessarily step up to 20W-50.
I'll say that I live in Arizona and on the '88, I do run 20-50 in the summer and 10-40 for the other 6 days of the year. Seriously, I run 10-40 when lows start to get below 75*F.
I Run 15w-40 motorcraft diesel oil in my 190k motor and it works great. Diesel oil is a much better quality oil and must put up with greater torture during testing before failing than gas oil.