+1 for a mechanical oil pressure gauge. I would use 10w30, thats what I use but if your trying to get rid of a ticking loose tolerances, like a leaky lifter) dont add something very very thin to it (Marvel Mystery oil), it just thins out the oil and will make the pressure go lower, if anything add something thick, like Lucas or I like the oil treatment stuff at advance, it looks like honey, thick as anything. Should help tighten up some clearances, helped with my tapping lifter anyway.