Use the red grease, lithium is a bit to thick and heavy. Basically with the forge DV it's:
1) open the DV up by undoing the top
2) slide the piston out
3) wipe clean piston and inside DV
4) you have 3 o-rings, two that sit inside the dv and act as a air seal between the outer casing and piston - renew if necessary
5) third seal is same as above - renew if necessary, this is located either on the top end of the casing or in the screw top
6) apply grease and reassemble
Not played around with a stock one for a while, it may be similar or it may be a solid unit.