Low oil pressure under load

[White Slicktop 87]

Hey guys I'm new to the forms but I'm trying to figure this out I've googled I've been I've searched multiple forms I cannot figure out what is going on I have a 1987 Toyota supra with an na 7M GE motor. Yesterday I had really low oil pressure so i stopped at my buddies house to pick some up because you have some spare. After i put it in at idol my sunpro oil pressure gauge was reading a little over five psi & on the way home, while under load it was nearly reading 0 psi. When I let off the gas the oil pressure went back up to five PSI not sure what's going on but the oil I used was 5W 30 could that be my problem? I slowly took the car home to avoid potential damage. Any ideas on whats happening will be helpful.