I have read conflicting information on lubricating a mainspring. Is it better to use grease or oil? What kind of grease or oil do you guys prefer? (I am asking specifically about pocket watch mainsprings, if that matters).

What about the inside of the barrel. Coat it with a grease, or not?

Tony,

For lubricating PW mainsprings I use Novostar Mainspring Grease. A 50ml bottle will last a long time. I've had good luck with it. I do lubricate a couple of spots inside the barrel. I also lubricate the mainspring by running it through a wax paper strip folded over with the mainspring grease. I don't pull the spring through the wax paper, rather I spiral the paper through the mainspring. Once lubricated, I put it on a mainspring winder and install it in the barrel.

Some prefer to install new mainsprings that are "pre-lubricated" with teflon without any additional lubrication. This is probably fine, but I tend to lubricate them first.

The mainspring grease stays in place well. Oils tend to leak out of the barrel over time.