Checking The 6 Essential Fluids In Your Car Engine Oil. After fuel, oil is the most critical fluid in your vehicle Coolant. With all the combustion and friction that is going on in the engine, it produces a lot of heat Power Steering Fluid Brake Fluid Transmission Fluid Windshield Wiper Fluid Vehicles are complicated machines, and keeping them running smoothly requires regular maintenance. A critical aspect of vehicle maintenance is monitoring the levels of essential fluids.