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Hello, I'm about to change gearbox oil in a series 3 1983 land rover. A manual specifies performance level A.P.I GL-5 or MIL-L-2105 and SAE Viscosity 90EP 80W EP.
I've seen GL-5 spec oil but as understood this may corrode bronze / yellow metal in the gear box. Is there a suitable oil / or supplier of a suitable oil please?

The Castrol EPX 80W/90 meets the API GL-5, 80W90 and MIL L 2105 specifications you are looking for. The EXP oils offer good corrosion protection to ferrous and non-ferrous metals, so this should not be a problem.

Hello Jack, I made an error in the query. The instruction manual specifies GL-4 not GL-5. My mistake as I raised this query because I'd purchased Castrol EPX 80w/90 GL-5 not fully understanding the difference between Gl-4 and GL-5. Can you re confirm if this oil is still OK please?

There are a lot of conflicting reasons why GL5 should not be confused as a replacement to GL4 but the main thing to remember is the gear oils were developed at different times. Transmission components are forever changing and therefore different oils are required. GL5 might cover the GL4 gear protection side of things but GL5 has about double the amount of additives in it which could affect the wear on other components like bushes. The safest thing to do is stick to the manufacturer’s recommendations to avoid any premature wear. If you are looking for a 80w90 GL4 manual transmission oil then the following should work for you; Valvoline 80W90 GL4, Shell Spirax 80W90 GL4 or Mobilube GX 80W90 GL4.

Hello Jack,
Thanks for pointing me in the right direction. ( no need to reply)