Neil,
just to add a little to all the good advice given here, The Stalwart gears oil is given as OEP-220, that is Oil, Extreme Pressure, thus it is an EP90, or EP80w/90. In the DEFSTAN specification book on lubricants, as I already quoted, this oil is not suitable where copper alloy bearings are used. If copper alloys were in the gear boxes, then 220 would not b specified. So to draw a conclusion, that it is a GL4 oil, but you could use GL5 if need be.
Just as an aside, as Karoshi brought it up, re. SAE90 and EP90, in a jeep gearbox there are bronze bushes and thrusts, so EP90 should not be used, also the extreme pressure qualities of EP90 affect the correct operation of the synchro cones leading to crunchy gear changes. Morris oils do a Straight SAE90 gear oil that works well in jeep boxes.
Richard