Replaced transfer case fluid (Auto Trac II) on 09/22/09 when my Envoy was purchased from a friend with 151,000 miles. It now has 193,000 miles. So that's about 13 years and 42,000 miles on the fluid. Is it time to service it again? 4x4 gets used about 6 times a year. 15-20 miles each time. I'm thinking just time wise it should be serviced regardless of the amount of use? Thanks for any input.