How often should one change synthetic oil in the differentials and transmission transfer case, in my SUV?

My dealer is at a loss, I'm the first to do it, about 40,000 miles ago. All works well, no leaks anywhere.How often should one change synthetic oil in the differentials and transmission transfer case, in my SUV?
Give the year, make and model for more info, please.


I can look up the service intervals then on several databases and give you the averageHow often should one change synthetic oil in the differentials and transmission transfer case, in my SUV?
I took my Denali in and was told it is at 70000 km is when u change the oils in your diffs and tranny.. At the lube place they show u what your fluids look like and compare it to what they Should look like.. Mine is an 02 and no leaks or problems...