Silicone grease is available at Home Depot, in the plumping
section. I've used it on O-ring when replacing the O-ring in
faucet and bath water valves.

I've never heard of rtv cement. I've used rtv silicone to
make gaskets. They're in auto stores.

Use silicone grease to seal O-ring on turning shaft. Use rtv
silicone to seal stationary parts.