Noisy lifters

My S-23 330cid DeSoto engine makes what I am sure is lifter noise when I drive up an extended hill. After a few miles of level or downhill driving, it quiets down again. It's just one lifter, and I believe it's got to be the front one. My local trusted mechanic says that for the amount I will drive this cart (not very much) this problem is in the "live with it" category. OK, I can deal with that, but I still wish it were quieter.

I'm running SAE 30 oil with about 20% Marvel Mystery Oil in it. The Marvel does seem to help. Dashboard indicated oil pressure is 15-40 pounds, a little low for my taste. If I want to attack this problem lifter without taking the engine apart, do I want higher viscosity SAE 40 or even SAE 50 oil to get the oil pressure up a little? Or do I want lower, SAE 10 or 20 to get it to circulate better? I know, I can try them both and see what works, but some experienced hemi guy on our site probably already knows the answer. Oh, frost and low temperatures aren't a problem in San Diego. We don't exactly have a winter here.

One more thing. I might be willing to remove the oil pan and replace the pump, if that's all it took. Anyone know if it's possible to replace the oil pump without a complete engine tear down?

-John

PS: Just got a new Hernando for the hood ornament from George Laurie. It's pretty. The 1936 S2 is progressing slowly. Transmission, radiator, and water pump are away to the pros for rebuilding.