Is there a good chemical to quiet lifter in my '51 Lincoln flathead

[Hoop]

"The noise appears only after the engine runs and warms. When cold, it's quiet and runs great. I have run lots of Mystery Oil in the crankcase, but no help. I'm now running fresh 40W non detergent oil."

Why Rislone? ATF? MMO? ... or any other thinner?

The symptoms suggest the lifter is pumping up when the oil is cold and thick, but when it warms and thins out, you get the noise. Sounds like a worn lifter.

Straight grade 40 thins out to 40 as it heats up. Cold it is thicker than 40 grade.

Why put thinners in the oil? How about doing a test oil change and put put in Wal-Mart brand 5W-20, or 10W-20 (or 10W-30) ... something less than 40, but multi-grade. It will start cold at 20 (or 30) grade (unless you're out in a blizzard in Rockport) ... it will maintain 20 (or 30) grade viscosity as it warms up ... and the worn lifter should be noisy from start up, or very quickly there after, since it's noisy at warm 40 grade.

Easy test.