I used a silicone grease, Molykote 111 as I use it for the o-rings in my air rifles.
Its ~£13 a tube but it lasts forever because you only need a tiny bit although I bought a tube for £6 off a guy who has to open a brand new tube every time they grease the o-rings on a machine at work and then usually throw the virtually full tube away.
Cheers
Stot