Hi, I've only been using my C5 for a short while but I used Teflon grease in the gearbox and all bearings. It works great! The C5 coasts really far and very smooth. You can buy it from Halfords for about six or seven pounds, look in the bike section and it comes in a white tube.
Finish Line Teflon Grease (DuPont Teflon)
27% lower wear rate
44% higher extreme rating than before.