The Oil Drain Plug. At the base of the oil pan is a drain plug, accessible from the underside of your car. Worn out thread, misaligned threads, or a loose oil drain plug can be a common oil leak cause. It’s easy to spot as there would be fresh oil around the plug, and on the side of it where it drips down.The Oil Filler Cap.hat Causes Oil Leaks. Oil leaks can be frustrating because they don’t come from one place on a car. If you have an oil leak, you’re almost guaranteed a trip to a mechanic. All cars develop them, from minor leaks to slow drips that develop into a major threat to a vehicle.