Changing the oil to shell rotella 15w40 increased the oil psi in my tractor. 1957 TO35 still has the original oil pressure gauge so it does not have numbers but the 15w40 did keep the needle in the center.
Others may kick in here, but that oil pressure level may be ok, you have to remember these old boys are not new tractors.
Might get a oil filter from the local Ferguson dealer if you are using a cheap after market filter.
Also might add 1 qt. Of Lucas oil stabilizer, this is great oil.