Oil gets into the clutch compartment of those tractors from the engine and from the transmission case. Can also externally find its way down through that hole in top of the housing where the pto linkage goes down, but a leaking steering valve or remote control valve will be the problem there. Rear crankshaft seal, seal housing gasket and oil pan gasket for engine oil leak source. Transmission input shaft seals are used to keep oil in the rear. Tractor requires a second split at rear of clutch housing to replace shaft seals. Oil on clutch disks is usually from rear end. 50 year old tractors usually have bad seals. Also always replace pto shift pins when tractor is split for seal replacement. Hope this helps.