I changed mine the first time at about 13,000 also, it looked and smelt bad. I changed again at about 30,000 I don't recall how it looked but it was not as good as when I just changed it at 60,000. Changing at 60,000 I had some 30,000 miles on the oil and pulled a 10,000 lb 5th wheel about 10,000 miles over the time the oil was in for that 30,000 miles. When I dumped that oil it looked like the day I put it in. I plan on putting about 40,000 miles on this change. As said there is no need to throw money away. My first 5 gallons of gear oil cost about $115 the second 5 gallons cost $185.

 

For the first time I would drop in ASAP as when I checked mine for the first time it looked like gray water with only 1500 miles on the truck. Now that it as Amsoil SVG 75/90 I'm sure it will last 50K being thats is what it's rated for under sever usage and IIRC 100K for normal. FYI, but not all gear oils are made the same so YMMV depending on what brand you use. BTW, the 2006 owners manual calls for diff change at 15K for sever usage Sch-B and 120K for normal usage Sch-A.

 

I didn't think it was 30k for normal usage etc.I have two buddys that towed 8k or so 90% of the time change theirs near the 100k for mark first time.They both said to them that was sch.A normal usage.The fliud came out looking like mine did at 15k and no unusal metal in the bottom of the housing.

 

Hmmm, don't remember for sure on the 30k normal service interval. Have to break out the owners manual when I get back home. Been working in Jacksonville, Fl. for the last 4 weeks Mon-Fri. Off to Atlanta next week for a week and then back to Jacksonville. I got a job, hope I get to stay in Savannah later.......