The small oil refinery boosts your maximum fuel. In other words, before you had it, you may have had a limit of say, 135 fuel. When you built the oil refinery, you're maximum would increase to 155. So you actually don't get "barrels" of fuel like you do with the Fuel Station.
This is the kind of thing that really peeves me about this game - thanks for the information.
Regards
Captain WH Rollins