General "rule of thumb":
Dry gasket: mount dry and clean
O-ring: light coat of the fluid it is exposed to (oil, brake fluid, gear lube, etc.)
Seals: light coat of the fluid it is exposed to on the seal's rubber lip
On engine work, a light coat of assembly lube may be substituted for oil based on keeping the engine external surface "clean", but the metal and sealing surfaces lubricated. But only if the oil will be changed fairly soon after engine is back in operation to get the contaminated oil out of the engine.
TQ