I run the Motorcraft Synthetic blend, or the Valvoline Durablend. I am running Synthetic Valvoline in the rear diff, and Synblend Valvoline in the front diff. Was running full synthetic Mobile 1 ATF in the trans'/tcase, but it allowed the syncros to slip too much. I am just running plain old NAPA ATF now, but I am going to change it for some Mobile 1 0w-30 soon. It does better with lubricating a gear box (like the M50D) than an ATF fluid can (since ATF is a hydraulic fluid).