I am about to change the transmission fluid in my transfer cases for the first time. Was told many things, first to do it at 7k miles, then to do it at 15k, then at 40k. I have 12k now, and think it just plain time!

Thing is, I can't find a local dealer who's got the "mix" to drop in the cases. I was told by St. Charles Isuzu (a reputable shop) that any blended transmission fluid (like for a GM or Dodge SUV) would work fine. Any ideas?

The guys at my local shop say they can't mix the additive into the oil; they told me to use a pre-mixed one (which they don't stock).