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 reservoir 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.
Downforce and lift are the names we give the forces that push straight down or up on the car. Downforce pushes the tires harder into the track, while lift pulls up on the car. These two forces are in direct opposition to each other. Whichever one is bigger wins. You want to maximize downforce and minimize lift.
The oil tank is open to the bottom of the car. Air under the car creates lift. Even though you try to keep the splitter close to the ground, there is always some air that gets under the car. If the oil tank lid isn’t firmly tightened down, it creates a path for air to get out of the car, which reduces lift. Josh told me that there may also be a drag reduction–there was with the old car, but he wasn’t sure about the new one.
On NASCAR Now Monday, Ray Evernham commented that having the lid off "allowed the rear wing to make more downforce". The total downforce is the difference between all the forces pushing down and all the forces pushing up. From my discussions with Josh, I assume Ray means that the open oil tank lid decreases the force of air pushing up on the car, which means that there is less lift. If the rear wing (or spoiler in the case of the Nationwide cars) produces the same amount of downforce, but the amount of lift decreases because of the loose oil tank cover, then the net downforce is larger because there is less air pushing upward. I can’t see how having the oil tank cover missing would lead the wing to generate any additional downforce unless the decreased lift changed the car’s attitude, in which case all bets are off.
One of Rusty Wallace’s cars originally penalized in the Nationwide series won its appeal on the basis that all of the bolts on the oil tank cover were engaged fully and the design of the oil reservoir was such that it led to the apparent opening. I can imagine (especially having seen graduate students over tighten bolts) that if you screwed down really hard on the bolts and the oil tank lid were on the thin side, you might be able to warp the cover on the oil tank lid a little.
The case of the No. 99 car’s oil reservoir lid is a little different, though, because the reports have been that the lid was entirely missing. Carl Edwards said on NASCAR This Week that a "bolt backed out", but it’s a little hard to see how losing one bolt would lead to the disappearance of the entire plate. Boris Said asked the important question on NASCAR Now Monday: "Was the lid in the car?" StockCarScience.com