I would definitely try a heavier oil. I run diesel grade in mine. With an extra quart added. It has helped increase the pressure in mine, however when it gets warmer it stills falls to between the the L and the line, then it will idle and it gets toward the N, or sometimes it is lower. but it doesn't knock. I changed the oil pump and the oil pressure sensor. If you are going to change anything try the sensor first it is much easier and just a bit north of the oil filter. (looks like a tiny oil filter with a wire in it.) I think with mine I didn't get the pickup tube tight enough, should have pulled the engine instead of doing it in the truck, I didn't have a engine hoist back then though!

It is an incredible engine though, I have seen them with over 300K in a service truck, driven by a twenty something trying to kill it, with it knocking all over the place, still chugging down the road. Simply Amazing.