Laycock was originally rather specific about not using EP gear oil, and in the early days, that is also what Triumph recommended. Around 1959 or 1960, Triumph switched to recommending only EP gear oil. The story I heard (supposedly originating from an ex-factory engineer) is that they were having too many gearbox failures under warranty using motor oil, and felt that using gear oil would better protect the gearbox. Apparently it worked, because they kept recommending gear oil through the end of the TR6 run.
Personally, I use Redline MT-90, which is a synthetic GL4 oil specifically designed for use in synchronized manual transmissions. Before that, I used Valvoline VR1 20W50 motor oil. OD operation seems about the same on either one, but the Redline made the gearbox shift noticeably better.
I'm not certain what the right pressure is; it's different almost everywhere you look. For example, the TR4 workshop manual says 490-510, while the TR6 book only says 410-430 and the Stag 480-500. I'm guessing that the right answer depends on what engine and rear axle you have. (The IRS A-types have a smaller accumulator, apparently to make them shift smoother and not shock the IRS so hard when shifting under power. Lower operating pressure would help smooth the shift as well.) There is also a service note from 1964 that says it varies "according to the installation". From my own experience with TR3/A, anything over about 360 seems to work well enough, but any lower than that will let it slip in 2nd gear. Also, the OD service manual was written when the operating pistons were smaller, and OD only available in top gear for the TR2.
Watch out for potential coil bind when shimming up the main accumulator spring (if you have the early large accumulator). The Buckeye articles mention this in passing, but IMO do not fully explain the problem. When testing at relatively low rpm on the bench, the accumulator piston only has to open the relief port a tiny bit to control the oil pressure. But when pulling at 5000 rpm (or beyond), it has to move farther. I managed to shim to the point where the spring would bind with the ports only slightly open, which looked fine on the bench but let the pressure go way too high at high rpm. Lots of damage, including ruining the clutch plate, breaking thrust washers, etc.
Hi Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
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