The M50D is a manual trans that requires a thin oil like Mercon III ATF. It requires the thin oil because it uses tiny needle bearings all over and thick oil will starve them. The M50D also has soft synchros that will burn up real fast with the wrong oil. Synthetic ATF isnt affected by changes in temperatures like conventional. Since a manual trans doenst pressurize the oil like an auto, it takes longer to warm up to oil, which means the oil is really thick so it starves bearings and synchros when its cold. Thats why synthetic is the way to go. It doesnt thicken up as much when its cold, so the bearings get oil with the proper viscosity right at start up. Although ATF is thin enough to get into the bearings it wasnt designed to be in a gear box (like the M50D) it was designed to be a hydraulic fluid. 0w-30 is a full synthetic motor oil that has a viscosity very similar to Mercon III, but it has much better lubricating properties. Synchromax is a full synthetic trans fluid made for manual transmissions have soft metal sychros and that ATF or motor oil.
