Bought a '65 L I and I made sure it ran before I disassemble it for cleaning & painting. The L pegged the oil gauge. When I get it put back together & can let it run longer, I want to check the oil stream going into the advance casting. I knew that someone on here (I think it was you) had discussed adjusting the oil pressure regulator if not enough oil was going to the PTO. None of my other L's or C's peg a oil gauge like that one did.To adjust the oil pressure regulator you adjust the length of assembly.
You loosen the oil line.
Loosen the jam nut.
Turn the assembly that the line screws into in for more pressure, out for less.
Tighten jam nut while holding main assembly to ensure adjustment stays put.
Tighten oil line.
The real question is why are you trying to adjust this? I recently reset one that a previous owner adjusted all the way to highest pressure (screwed the assembly all the way together tight). While this pegs the oil pressure gauge on the high side it also cuts the oil flow to the front planetary, clutches and the shifting dog. Most consider the oil pressure a good indication of the condition of the bottom end of the motor. Gravely service manuals say this adjustment is factory set and should not be messed with. If you are looking to up the oil pressure to the main bearings note that this comes at expense to the rest of the tractor transmission.
If this has been disassembled or clogged requiring cleaning.. by all means adjust it! Just make sure you maintain a good oil flow to the front end of the transmission and the attachment slip clutch area.