differential oils may not have the right additives to prevent yellow metals wear in the bearings of the gearbox. GL-4 type additives are necessary.

If an 80/90wt fluid is necessary, the best IMO is Amsoil MTG.

http://www.amsoil.com/storefront/mtg.aspx

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)