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API specification GL1 is basically almost straight refined mineral based lubricant. It's designed for slow moving low pressure gears and worm drives like tractors and really old truck rear axles. I don't think they put much of any "Extra Pressure" EP additives in it. GL3 with mild EP additives was obsoleted by GL4 many years ago but GL4 still a good gear lube for older cars with non hypoid gears and phosphor bronze type gear bushings. GL5 has EP amounts high enough to work well with hypoid type gear sets and high pressure gear sets. The acidic quality of the EP additives can be harmful to phosphor bronze gear bushings. There are other ratings too but they are mostly for specialized rear axles with limited slip type clutch packs in them.
https://www.engineersedge.com/gears/...on_general.htm