"Hypoid" lubes are intended for rear axle gears with the pinion gear intersecting the ring gear below the centerline of the gear. If you turn one of these units and watch the action, you can see that there's a wiping action of the gear teeth, instead of a straight gear-to-gear contact like with a spur gear. The extreme-pressure additives that allow the lube to cling to the gear teeth instead of getting wiped away have a tendency to carbonize on a synchronizer ring. It's not too much of a problem on a lightweight vehicle like a pickup, but on an 18-wheeler moving several tons of cargo, it can be critical. Fuller advises against using hypoid lube in their Roadranger twin countershaft transmissions, to avoid filling up the grooves in the synchronizer rings with carbon deposits. I've never seen hypoid lube cause problems in a light or medium-duty truck tranny, though.
Jerry
