I have a pair of Dodge WC 3/4 tonners, the WC51 and the 54. I use striaght 90 in the diffs and box and 20/50 oil in the engines. I am (JUST!) old enough to remeebr the fanfare when modern cutting edge 'Multigrade' engine oils were introduced. My experience is everything runs on the old straight oils, within the limitations of the oil. The 90 and 20/50 just make life easier, especially after long runs the engine oil pressure stays higher and recovers faster, the transmissiom as said has less drag. Engine oil my current one is the Halford's Classic 20/50.