Hi Andy,
At one time, EP gear oils were not "yellow metal friendly" and bedford boxes having bronze bushings in the gears, this is likely why they only specified a Straight 90 gear oil.The army did not have that grade in the inventory, so substituted it with Straight 50 engine oil ( the viscosity is nearly identical as gear and engine oils are worked out differently). It may be that in the late 1990's, the OEP220 spec was changed to a GL rating that was compatible with yellow metals. I would tend not to use extreme pressure gear oils in a synchro box as it inhibits the braking action of the baulk rings, this is why Jeep gearboxes perform better with a Straight 90 gear oil, rather than an EP90.
I know the RL, MK and MJ were using OMD330 in gearboxes, but not sure when they might have changed.
Yes there are some bronze bushes in the gears.
regards, Richard