Ken,
I use 30w non-detergent NAPA oil.
Then in the gas for each tank full of gas I add 1 (8oz) bottle of Lead Substitute from STP (also sold at NAPA), and 8oz of Marvel Mystery Oil,
(also sold at NAPA). A sort of Cocktail if you will in the gas that makes my
1948 Chrysler Royal (250.6cu.in.) 6 Cylinder Flat Head run like a sewing machine. It only has 53,900 original miles on a never opened engine.
If I rebuild it, which I hope to do in the next 5-10 years before I retire
I will probably start back with 20w oil just like the Service Maintenance
Schedule calls for in the book. I owned a 48 Royal in the 70's and drove it as an everyday car. The rings went, and I rebuilt it myself and used 10-30w oil
then the rings went out again every 10-15,000 miles.
Now that I know engines better I believe the sleeves had excesive taper, thus my problem, but alas I sold her in 84. I think straight weight is the way to go.
Tom