No, most gear oils are GL-5. GL-4 is used in applications where bronze components are utilized. GL4 and GL5 denote the EP protection level of the gear lube. Both have EP additives and AW additives, just different treatment levels.
Most differential lube manufacturers list the GL5 rating because the majority of differentials require that protection rating.
GL4 is sometimes specified for components such as TC's and MT's because people have found that GL4 offer a less aggressive "attitude" toward non-ferrous parts such as the copper alloys of brass and bronze commonly used in sychronizer assemblies.
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