I run shell rotella 15w40 diesel oil in my old TO-35 with z134 engine.
this oil maintains better oil pressure and also has the zinc additive that oil had in it back it the day.
you can also run a qt lucas oil stablilzer in the oil, this will also help increase the oil pressure in a tired old engine. It's good stuff.
I just run the shell rotella because of the zinc additive.