The extreme-pressure additives in rear end grease can have a detrimental effect in some transmissions, but I don't think anything smaller than a Fuller Roadranger will show much of an effect. The shearing effect of the spiral teeth on the ring and pinion gears in the rear end creates friction that the film strength of plain oil cannot handle. The Roadrangers I used to maintain needed to use "straight mineral oil" as we called it, without the hypoid additives. Hypoid lube had a tendency to fill the grooves in the brass synchronizer rings with carbon, for some reason. If I tore down a Roadranger, and the synchro rings were black, I knew it had been running EP 90, not mineral oil.
Jerry

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