Synthetic oil or "energy conserving " dino oil in the primary is not a good choice here in big twins. They contain friction modifiers and a clutch with some miles on it could be prone to slippage from using these oils.

The synthetic oils are a great choice for the engine as they will reduce friction between the moving parts, this however it the opposite of what the friction disks need in a clutch.

Most 10/40 or 20/50 without friction modifiers will work fine or you can use the HD formula + lubricant in the primary.