@Jerry Bransford why do axles run cooler?
It's the ring & pinion gears that they discovered run cooler when lubed with a conventional gear lube. Surprisingly to everyone, synthetic was acting more like a thermal blanket and it wasn't sinking (extracting) the heat out of the gears as well as a conventional gear lube did.
What fooled everyone for so long was that the diff covers were cooler with synthetic lubes inside when their temperatures were measured with an IR gun. It turns that that was happening because the synthetic was holding the heat in the gears more and not extracting it. They only figured that out when they figured out how to build a jig so the temperature of the gears could be measured directly.
It's counter-intuitive when you measure the temp of a diff cover holding synthetic and see that it's cooler, which led pretty much everyone to believe the synthetic was causing the gears to run cooler. In fact it was the opposite and it took a lot of testing and re-testing to convince everyone what was really happening.
Companies like Currie were seeing more R&P failures in their custom axles than expected and it was trending up. It turned out that was because more and more were starting to run synthetic gear lubes which they hadn't thought of at the time. Just that more clients were reporting they were using synthetics which alarmed no one at that point. That's when the investigations started and it took a while to figure out what was really happening. John Currie shared that with me at a local 4x4 show and he said he was as surprised as as everyone. They changed their warranty to require a conventional gear lube in their axles to maintain the warranty and their warranty issues went back down to the expected level.
For normal axle use it doesn't really matter if you run synthetic or conventional. But for gear break-ins of aftermarket gear sets that aren't broken in and axles in hard use like Currie's are, it does make a difference.
I had a set of brand-new RGA gears in my rear axle fail two years ago after a very conservative break-in. Not trusting myself and the friend who did my regearing to install the replacement gears again with that kind of problem, I took my Jeep to one of the best gear guys in SoCal. As soon as he got my gears out he said the gears had gotten too hot and said 'you used synthetic gear lube didn't you'. I had. He told me why not to run a synthetic in my axle, due to my harder than typical street uses, and said especially not to run a synthetic gear lube during the gear break-in. I took his advice and that next set of gears did fine.
Street vehicles do fine with synthetics and so do new vehicles since their gears are pre-lapped before installation and thus need no break-in. Aftermarket gears? I'd use nothing but a conventional gear lube in a Jeep that sees much hard use.