I put 4:10 gears in mine about 11 years ago, use synthetic and only have about 110,000 miles on gears. Also synthetic in transmission and engine. All Amsoil.
Runs much cooler.
I have changed/flushed brake fluid several times, changed/flushed radiator several times.
Just had to put in new radiator/hoses/thermostat/belts/tensioners and water pump. Also did A/C.
Change engine oil at 9 or 10,000 miles.
While I obviously have deep respect for your opinion and experience over the years with many aspects of what works and what does not, You and I are at odds with the lube of choice going into the rear end.
Like you, many years ago, I swapped out the rear gears in my '39 Chevy streetrod to 3:73 and added Mobil One Syn Lube. It was suggested that it get changed after a few thousand miles and I never did do it. Did I have a problem..........not really, but I noted a increasing whine that was not there early on.
Bottom line is I highly respect the opinion of the owner of EAST COAST GEAR SUPPLY. He has a business in Raleigh, N.C. and has written an article regarding the right choice for gear lube in rear ends. I cannot with any accuracy describe the real reasons that a lube that is NOT Syn. is better than those that are.
It had something to do with the fact that syn oil/lube is thinner (less viscous) and lacking in "shear"???? capability..........I may have that factor wrong, but bottom line is this guy is in the daily business of providing rear gear swaps to mostly off road scenarios like Jeeps ,etc. and he has the experience/knowledge to know what works best. It is "key" to his business as he backs up his services and wants the best results for his customers.
I would love to get a copy of his (article) regarding why NOT to use syn. lube in a rear end so we could all understand it better.
Stay tuned as I will try to get it.