i run 10w30 valvoline with 1 qt. of lucas oil stabilizer and wix filter.
have 91K miles and it still runs fine.
is water level ok, be sure you are not losing water into oil or like mine the intake manifold gaskets were leaking into engine.
might also try a can of restore both these products helped keep oil pressure up on my old tractor
many use shell rotella 15w40 oil
any of these are worth a try, cheap fix, before you pay for a motor rebuild