Hi,
I am well acquainted with both Saracen and military spec of oils, having worked for the military when Saracens were still in service, here is the full works;
The spec OEP220 is a British Army designation for 80w/90 gear oil, in the full description of this oil it says, I quote "This oil may not be suitable for use in systems containing gears or bearings made from copper alloys". Now as OEP220 was the listed gear oil for Saracen, Ferret, etc, (except in the Wilson gearbox, that has SAE30 engine oil), this tells you one thing, THERE IS NO YELLOW METAL in the Saracen drives, so you are free to use GL4 or 5 without issue. For anyone else, if the army spec was to use OEP220, then there is no issue regarding copper alloys.
To further help our US cousins to find the correct oil grade from their local suppliers, the NATO designation for OEP220 is O-226 and the US military designation is MIL-L-2105D 80W/90