This from Stock Car Science:
The Science of ...Missing Oil Tank Covers
Danny LaDue asks: Can you explain the location of a NASCAR oil tank reservoir and how the lack of one could improve aerodynamics?
Thanks for the question, Danny. I can–with a little help from Josh Browne, Chief Race Engineer at Red Bull Racing, who helped me disentangle a couple of things I heard on television.
First, the location. Unlike your car, the oil in a NASCAR car isn’t stored in the engine (which would be a wet sump system). NASCAR uses a dry sump system, in which oil is stored in an oil tank reservoir. The oil tank reservoir is located behind the driver’s seat and is surrounded on the sides and top by sheet metal, which forms the oil tank. You can see a picture of the oil reservoir in the Stock Car Science Building A Car section. The sheet metal plays an important role in minimizing heat radiating into the car, traps fumes from the hot oil, and serves as an additional firewall. This function is so important that NASCAR doesn’t allow the top of the tank to be attached using quick connect fasteners. The teams usually duct tape the lid on. The picture below shows the location of the oil tank with respect to the chassis. It doesn’t show the cover, which would sit on top of the tank. (added) The oil reservior itself is closed and pressurized.