There would be one way of determining the quality of the factory oil although probably more than a typical oil lab test, and that would be to draw out some oil from a new truck and send a sample to a lab that could do a more elaborate test of the oil. The issue with too little oil in the diff may not be the lack of lubrication of the diff bearings and gears themselves ( although a lack of oil volume for cooling ) but the wheel bearings because at some point the oil would be too low to properly get onto the spinning axle or fed along the axle tube. That was the claim by the local dealer from a couple of trucks in recent years that had the wheel bearings fail and they figured from lack of sufficient oil due to a severe underfilled diff and some of the bearing material made its way to the diff and it got damaged as well so the axle housings were just replaced on warranty. But your right that if the diff is over filled by whatever margin that it causes more churning of the oil than is desirable and that is no good either and can cause a pinion seal to leak. Also old oil I believe can tend to loose some of its properties like antifoaming and another good reason to change the diff oil every so often.Ā
I would be surprised if the diff's were not filled (with the cheapest gear lube) at the axle factory before being shipped to GM.Ā If you ever watched them building trucks they install the axles and all suspension parts with the frame upside down and then turn it over before its time to install the engine.Ā Ā
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Too much gear lube in a axle can be worse than not enough especially with a lower quality GL where is get whipped up with entrained air (foam)Ā weakening its ability to lubricate.Ā Ā
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