Just to be clear, the old Hypoid 90 gear oil was required to be compatible with copper, where modern oils may not be.
It’s part of the specification in the Handbook of Service Lubricants March 1945.
As of the 1950s, Hypoid 90 or GO-90 becomes OEP-220 but is still required to have no delitirious effect on copper.
The spec for OEP 220 has apparently changed over time, and now says that it may not be suitable for use with copper alloys, even in a GL5 spec -
https://www.dstan.mod.uk/StanMIS/Ind...945?seriesId=2