Every 15k is a waste of good oil. I clean the magnet at every oil change. The diffs at first 15k then at 50k and every 50k after that.

 

I've been using Mobil 1 75W-90 in my rear diff. That's what the manual calls for. Today is the first time I've ever seen anything suggesting to use 75W-140.

 

I just changed oil in both diff's at 20,000 and think it was a waste of 50 bucks. It still looked new and only had a very small amount of grey grit on the fill plug. I went with mobil 1 75/90.

 

My neighbor on his first 3rd generation truck a 03 6 speed 3.73 geared went 90k on all fliuds but engine oil of course.The diffs,trans,transfer oil drained at 90k looked really clean.He traded it right before his 2nd 90k and all was well.