I'd always used gear oil and didn't even think of using Dextron in my transfer case, but noticed an annoying leak at the output shaft that was causing WAY premature drying of the ujoints and had to replace them every thousand miles or so even if I regreased them daily. transfer case was new enough to swap over to using Dextron as it calls for, flushed it using Dextron for about 5 cycles till there was no more grease noticable in the draining, and have run Dextron since, and no leaks and ujoints are lasting about 10+ months or so now. If everything is functioning normally and you don't have any side affects of using the gear oil that you have been using all along, stick with it. If it ain't broke, don't fix it!