I have encountered knocking fuel pumps. When the rocker-arm pivot pin gets loose they will knock, aside from leaking oil.

Twenty/fifty motor oil is a heck of a lot thicker as you pour it out of the bottle than is 20/20w. If you're not working the engine hard I would stick with the factory-recommended 20/20w, if you can find it; straight 20 is probably fine. You don't say where you are but unless it gets very hot I'd stick with SAE20. These old engines ran rather tight clearance on rods and mains and quite low oil pressure (15psi) hot and running. That's not enough to push much oil through if it's thick oil.

You say clearances were fine 15,000 miles ago and unless you've abused the engine in some way I wouldn't think it would have changed since then. I would just run it and see if the noise returns or gets worse.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.