Oil pressure was normally on the low side for these tractors, with anything over 5 psi considered good. I don't think cleaning the line going to the gauge will help. A plugged line may slow down the response of the gauge, but once the pressure has built, the gauge should show it.

On the right side of the block, below the oil filter is a plug that looks like a bolt head, with a copper crush washer under it. Take that out and there will be a spring with a plunger. The plunger could be stuck, or the spring could be weak or broken. Either could cause low oil pressure. I'd double check the pressure with a reliable gauge before I started buying any parts.