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The primary shares fluid with the tranny, but they are two separate assemblies. The primary houses the sprocket coming off the engine, the chain that transfers power from that sprocket to the clutch, the clutch itself, and the alternator assembly. At the bottom of the primary there is a fluid passage to the transmission. I have used Sport Trans fluid with decent results but I have noticed an improvement in shifting with other fluids.
Since the fluid must provide lubrication for the transmission, I would not use an engine oil since it will quickly shear down. A 75W-90 gear lube or manual transmission lubricant (MTL) is basically the same viscosity as a 20W-50 engine oil yet is made to resist the shearing of the transmission gears. I have seen UOAs of 20W-50 that has sheared to a thin 40-weight oil in less than 3,000 miles when used in a Harley transmission.