New gears, new ARB, which gear oil..??

[Mark]

gawd.. you'd think I'd find a clear answer in search, but I'm seeing contradictory answers...
Have new 4.10 gears in my 9" rear with a new ARB.. my conventional thinking was dino (non synthetic) gear oil. But what weight?
how many miles should I run it, drain it, refill it with dino or synthetic again?
How many miles before I can call it broke in?

 

[okorangebrnco]

Amsoil will run a little cooler, but will seep through a gasket/RTV over time. Standard 80-90 will be great just the same. To work harden the gear and new bearings usually runs in around 350-500 miles through at least 2 good heat cycles.

 

[broncosbybart]

80-90 is fine for break in. Do not use synthetic to break in. Initial use should be only a couple miles (literally) under 50 mph. Then let it cool. The dif will get warm. Do a few cycles like this, til you get to about a 15 mile run. No hard accel.

After that, you can drain and replace fluid with whatever floats your boat.

 

[Broncobowsher]

If the diff is getting warm in only a couple miles it is built wrong.
But I like the conventional oil, give it a few months and a few highway trips. Maybe even toss a small trailer on after a while. Get a few thousand miles on it. The magnetic drain (or fill) plug should get a nice little fuzzy beard on it. After that change to the oil of your liking.

I was a 75W140 fan. But the more I get to know gear oil I like the 75W90 better. The 140 is more of a patch for towing with an undersized axle, my thoughts anyway. There should be no reason to boil 90wt in a Bronco. Low speed stuff is so slow you never warm up enough to get into the advantages of the 140.