What do you lube the thermostat gasket with?
You do not "lube" the gasket with anything. If it is a flat
paper gasket, you can use any shelac type sealer, but you don't
have to as these will usually swell with contact with coolant and
seal. If it is the kind that is a rubber ring around the thermostat
itself, these require no sealant as pressure alone seal the housing
to the motor.