Our propane water heater vents through a 3" PVC pipe on top of the
unit.
It look as though a bead of silicone(???) was placed around the area
where the PVC piping connects to the vent line from the heater itself
(it was installed about 6 years ago).
I have noticed that this seal has sprung a very small leak and when
venting, some of the air is blowing into the surrounding area.
Obviously, I would like to reseal this connection.

Is a special purpose silicone used for this seal?
The instruction manual says to use a high-temperature silicone for the
seal, but no local stores seem to have heard about this type of
silicone.

Is the high temperature necessary due to the heat from the venting
itself?

THANKS