ZuDfunck, Whats A Oil Reservoir Cap?

Glad you asked. I have images of what it looks like on an old car courtesy Stock Car Science but I think it is basically the same on the new as far as the advantage it creates her blog has this:

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 tanktank. 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.

So if the oil tank cover plays such an important role, why would you leave it loose, much less leave it off? The answer is aerodynamics. The air exerts forces on the car in different directions. We give different names to those forces depending on the direction in which they act. Drag is the force air creates along the length of the car. Air creates drag when it hits the front of the car, but it also creates drag when it gets inside the car because there is no way for it to get out. Drag always acts opposite the direction the car is trying to move, so you want to eliminate as much drag as possible.