The difference is in the design of the gears. They're called "hypoid gears". The pinion gear in a drive axle is below the centerline of the ring gear, and the teeth mesh with a shearing action that requires a high pressure lubricant. The hypoid additives in some gear lubes supposedly can damage brass synchronizer rings. A rebuilding depot for Fuller Road Ranger transmissions that I used to do business with in San Francisco had 55-gallon barrels full of scrap synchronizer rings with heavy black deposits on them, and the manager of the facility told me they came out of transmissions that had been running "multi-purpose" gear lube with hypoid additives in them.

https://www.motioncontroltips.com/hypoid-gearboxes-what-are-they-and-where-are-they-used/

Jerry

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