Some of the older big rig rear ends used an outer grease seal that fit between the axle flange and the hub bearings. Those hubs ran grease-packed bearings. If there's no oil seal outboard of the bearings, then the hubs are lubed by differential oil. The level check plug on a rear end designed with oil-lubed hub bearings will be slightly above the bottom of the housing tubes. After installing new bearings, I would always jack up a wheel 3-4 inches off the ground to allow lube to flow down the axle tube to the hub and flood the bearings. Putting a little chassis grease on a replacement bearing was a good idea to assure there would be no damage during the initial run-in.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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