Apologies for the thread revival.
And before trying the oil change/synchro/box rebuild try this:
Sit in neutral with the engine ticking over, and whichever gear causes issue when trying to shift into it at high rpm just slightly push the gear stick into the gate of the gear causing issue...
Yeah I use anti cease on the threads, it's just something I herd on the www's regarding warm aluminium stripping.
I herd its more important to have an equal range between cylinders than it is to have high numbers as it showed even wear and that there isn't one specific cylinder that has issues...
Even then it wouldn't explain why he's getting a lot less, I didn't have my throttle open and achieved 220 psi, but it will help diagnose possible reasons why it may be lower I guess.
I know people say it depends on the gauge but I would imagine it cant be that different between brands...