What should I use to lubricate a freehub?

Freewheel (old technology) or freehub (current technology)? People tend to use the terms interchangeably, but they are quite different. A freewheel can be pretty effectively lubed with any medium-viscosity oil. There's not much in there; a couple of ratcheting "pawls" and their pivots.

You don't want anything that will gum up and cause the pawls to stick.

On a freehub, the mechanism is pretty well-sealed and greased from the factory. However, they are pretty easy to disassemble in most cases. They do make a clever tool called the "Freehub Buddy" which lets you inject grease into the thing without disassembly.

However, at 40 bucks it's a bit steep for home use where you would only use it every couple of years or so.

For a real freewheel see (@M. Werner), you want the lightest oil you can find. Never use WD40. Synthetics can work too. Anything heavier, and (especially in cold), the pawl springs can't snap back fast enough to create an ratchet.

As a bike wrench in the time when freewheels and freehubs both existed, I saw too much oil/grease more often than not enough.

If it is loud, just add a bit of light oil and move on.

since you commented that it freewheels perfectly, all you need is some chain oil in there as mscantland said.

any oil really (just not wd40!). general purpose oil will work, like 3-1. whatever you have around.

to apply follow my "option 3" from the other answer. it will help with the noise. which is totally 'cosmetic' if it freewheels nicely.

I have had great results using ordinary engine oil.