Appropriate oil viscosity for the inside of a freewheel body
The only freehub-specific lube I have found is from Dumonde Tech. They offer a freehub lube in both oil and grease form; depending on your preference. Those are what I use when servicing my rear wheels. Cleaning with WD-40 isn't bad practice as it's just a water displacement solution (Thus the "WD") and only imparts a very, very light lube to whatever surface it covers. I use it for cleaning bike and motorcycle parts all the time and to add some protective covering to parts with exposed raw metal.