You could just disassemble and clean the lifters and see if they quiet down.

The most important thing if you are replacing them is to coat the lobes and bottom of the lifters liberally with cam assembly lube and then assemble and rotate the engine a couple of times to verify that all the lifters spin. They should all turn like they are gear driven together at the same speed. I believe at least 95% all of cam failures happen because a lifter or lifters don't rotate consistently on the cam.

As for oil, you can pay the $$$ for the flavor of the month boutique oil if you want but if the lifters all spin as they should, any 15w40 diesel oil be it Rotella, Delvac or Walmart brand will be more than sufficient IMHO.

Kevin