Might try a bottle of Lucas oil stabilizer, that material would help increase the old pressure on my to35 when hot. At the time I was using 10w40 weight oil.
Currently I run shell rotella 15w40 oil. The reason I changed oil weights: I determined that the shell oil has the additive zinc and the car/truck oils do not have zinc.
The old engines need the zinc additive for long life, plus with 15w40 the oil pressure stays up in the normal range when the motor is at operating temp at idle.