I have been a strong proponent of conventional gear lubes for years, after reading of tests that finally proved our gears run cooler when lubed with conventional gear lubes. In fact many well-known axle builders like Currie require their axles run only on conventional gear lubes to keep their warranties in effect. Gear manufacturers like Revolution Gear specify ONLY conventional gear lubes in their instructions.
But because I drive a Rubicon I let myself get talked into using a synthetic gear lube during the break-in of new gears last year. My new gears started a horrendous whine 900-1000 miles later, on the way back from Death Valley. Revolution Gear sent me a set of replacement gears under warranty and I took my Jeep to well-know expert/guru of regearing. As soon as he pulled my gears out he became very agitated and exclaimed they had been overheated and accused me of having used a synthetic gear lube. He was right, I had... against my better judgement. He made it very clear that he has seen too many issues with overheating in axles filled with synthetic gear lubes and made sure he made his point to me to only use a conventional in the future. When I said I had been told Rubicon axles supposedly had a different lube flow design that required a synthetic, he laughed and said that was a bunch of "bullshit".
Not to mention that the gentleman who fixed my gear problem (Martin of Mobile Gears) is probably the most respected R&P gear guy in SoCal, even Revolution Gear was happy when I told them who would be installing my new R&P gears. Martin set up the new gears, refilled the axle with a conventional 85W-140 and life has been good ever since.
The first time I was told about the conventional vs. synthetic gear lube thing, it was during a face-to-face conversation with John Currie maybe 15 years ago. He told me how they were starting to get an unusually high number of new R&P installation failures... gears THEY had installed and knew they had been correctly installed.
Currie then figured out the failures were only with axles the customers had filled with synthetic gear lubes which at the time made no sense. From the damage to the gears, they knew it was heat related... as was with my failed gears. They used an IR temperature gauge to measure the diff housings and observed that the housings filled with synthetics were cooler than those filled with conventional gear lubes. So they figured it can't be from the synthetic gear lube. Months later with more failures and repairs being taken care of under warranty (it's getting expensive for them) they still felt they had a heat problem. So they built a test jig so they could measure the heat of the gears directly. What they found was, when measured directly at the gears, the gears running in the synthetic gear lubes were running significantly hotter than those lubricated with conventional gear lubes. That shocked them since the common knowledge back then was gears lubricated with synthetics run cooler. The research finally figured out that the synthetic gear lube is not as good at extracting the heat out of the gears as a conventional gear lube is. The diff covers were not getting as hot when a synthetic was used because the synthetic wasn't carrying the heat out of the gears and to the diff housing and covers so it could radiate out.
Once Currie and other high-end axle builders figured that out, and they were all shocked at what they had learned about the use of synthetic gear lubes, they immediately changed their instructions to say to use only conventional gear lubes. After that their warranty repairs dropped back down to where they were before their customers started using synthetic gear lubes.
I know some people and shops claim synthetic is ok but when I read Revolution Gear's instructions, Currie's axle instructions, and hear a gear expert yell at me after he sees the condition of my newly installed gears last year, all agreeing to run a conventional gear lube in my axles, I'm back with my strictly conventional gear lube mindset. I don't care if it's a Rubicon axle or a non-Rubicon axle.
