Recently I built a 99 silverado rearend with a Yukon locker and US gear for 4.56's

Break in went well!!

I did break in with Royal Purple....kind of disappeared on me in the rearend

We changed it out during the drain interval.

I put in Valvoline 80/90...since I did not like the Royal Purple just disappearing!!

It dirtyed up pretty quick...like at 6000 miles and I changed it again..

The gear does build up alot of heat...like low gears do.

I do alot of highway driving...speeds from 60 to 75....

Question"

Why is the oil breaking down so quick?

 

Are you having an oil analysis done to show that its breaking down? or just going by the color of the oil?

 

Just the color and how the locker operates..

 

How heavy is your truck?

I would run the thicker oil in there for the first 10,000 miles, 140w oil, I have noticed that royal purple is thinner then other oils.

with the 4.56's you are spinning the pinion bearings alot faster and you have slightly less tooth contact, maybe this is creating more heat then what a stock set would, but as long as the gears look good, no noise, and no leaks, i would just re-fill it and not worry about it until the next oil change is due

 

new gear oil was probably dark because of the RP. oil in a sealed system doesn't just disappear. gotta be going somewhere, pinion seal, axle seals. they're not always apparent. 4.56's will spin faster than milder gear ratios but shouldn't produce so much heat that it breaks down your gear oil. heat like that would only be caused by too much bearing preload under normal driving circumstances

 

Its about 4000 pounds...no noise from the pinion bearings