If it came with synthetic from the factory I would stick with it, but if someone put it in later leave it there, It will last longer then conventional. I changed mine (both diffs,trans, & t-case) to Red Line Synthetic at 100,000 miles and I probably won't change it at 200,000 when it rolls around. I allways ran Dura-blend in the engine and my boy used Max-life once and now it uses a little between changes if I don't use Max-life. I will turn 160,000 next week but with the 31s it will actuly be about 176,000 and still solid as a rock, I couldn't say that about the 2000 ZR2 blazer I had at 30,000 miles and that thier add campain <img src="/forums/images/graemlins/lol.gif" alt="" /> <img src="/forums/images/graemlins/lol.gif" alt="" />.
Dennis