I've read a bunch of stuff about selecting a good oil for older flat tappet engines. Some of it I understood and some of it I just hoped I understood.
The bottom line is that oils with the Starburst Oil Symbol are approved for modern engines with roller or sliding tappets.
Avoid any oil with the Starburst on the label and you will be OK. That
does not include motorcycle oils or lawn mower oils as they have totally different requirements and never should be considered for automotive use.
I have been using 5W30 Quaker Defy.
90% of engine wear occurs in the first couple of seconds when you start up a cold engine. Oil pressure senders are always located near the oil pump. That means the oil gauge will show pressure before the 20W50 molasses is half way down the oil galley. The heavy oil will also force the filter by-pass valve open, sending unfiltered oil into your cold engine.
If you want to use 20W50 you should have big racing clearances and a pressurized pre-oiler.