I think you need to do the pressure measurements again. P2 (Direction Clutch) pressure should always measure around 55psi less than P1 (Speed Clutch) pressure. The specification for P1 is 370+/-15psi with the oil at normal operating temperature, the transmission in 3rd speed forward or reverse and the engine at full RPM. Therefore P2 should be somewhere in the region of 315psi. Both those numbers are way higher than what you say you have measured. I'm also doubtful of the location where these pressures were measured because you mentioned "the back" yet the pressure taps for P1 & P2 are on the front side of the transmission under the cab floor.
The pressure at the transmission filter housing is really not relevant but in any case it should not be as high as 200psi otherwise it would blow the filter apart. If you want to measure transmission lubrication pressure the tap is on the front of the transmission directly below the taps for P1 & P2. With oil at normal operating temperature and the engine at High Idle the specification is 19+/-4.5psi and at Low Idle it should be a minimum of 1.5psi.
If you would like a complete copy of the testing procedure which includes the full range of both transmission and torque converter pressures then drop me a PM with your email address.
From your posts it would seem as though what you are being told (by the mechanic?) appears to be completely at odds with the machine test specifications. I think you need to either change your mechanic or get hold of the right information and so be the one telling him what to do rather than the other way around.
You operating symptoms give me every indication that you have an internal leak somewhere which will obviously increase as the oil gets hotter and its viscosity falls. The fact you have replaced so many clutch piston seals (and even though you are using the wrong oil) could potentially indicate wear in either the clutch piston housing or maybe even the piston itself, or both. I am assuming that is the seal you refer to when you said in your OP ..........
It has damaged the 1st gear ring seal 3 times now
......... correct..?
Lastly you are using an incorrect oil. The SAE30 viscosity is correct but the oil should meet Caterpillar TO-4 specification for use in powershift transmissions which contains additives compatible with the friction materials used in hydraulically-engaged clutches in a transmission. Normal motor oils do not have said additives and therefore do not meet TO-4 specification. This could (and I say that advisedly) be the root of all your problems but it might take something a bit more complex than an oil and filter change to put things right now they are so screwed up.
