I picked up an open bolt stock and onemenace let me use some of his teflon grease on my gun and it has worked FENOMINAL since. The open bolt system has done me wonders especially after seeing the maintenance and care from seeing others with closed bolt versions.
There part of the bolt that hits the bearing/roller of the hammer a lot which will initially wear away at the thin coat of paint on the open bolt system so I'd say put the grease on after the paint wears away (will only take a few hours while in a game to have it gone).
Ohh.. and dont forget... you'll also need to change the top part of your mags to fit the open bolt system. link available
here. You can also get em from Evike, but they take fucking forever to move forward with their orders.