they used to be just plain old red oxide primer . I'm sure it's not a very expensive coating
My oil tank, which is inside the basement, has the plain old red oxide primer on it as you mentioned.
The oil tank at my dads house, which was the same size and shape as our tank, and also installed inside the basement, had a thick silver textured coating on it that looked like the coating of the tank the OP is showing.
His was supposed to go into the ground outside but he decided to put it inside the basement for some reason. That was in 1954 and that oil tank with the thick silver coating was still in operation in the house when we sold it in 2019.
Maybe that thick black coating was meant for an outside tank like MushCreek said, that ended up being installed inside like my dads.
Jim