i have ran 5w-30 full syn for the last 80k, only filled 2 quarts. i rebuilt my t-case about 25k ago due to the shift fork collar and wear fingers being worn out after 190k (common issues). inside was very clean. rear tail bushing is still tight, no seal leaks, chain has minimal stretch (best i can tell, no spec for the damn thing), planetary looks great, and i only replaced one bearing (rear output bearing had some notchyness to it i didnt like). Put 5w-30 back in, filled with 2 quarts and no issues since. checked this last summer and still looks new as far as oil look. still full too