Derale Fluid Control Thermostats are highly recommended for both engines and transmissions. As the fluid temperatures exceed 180°F the thermostatic valve closes, allowing 100% flow through the oil cooler. At temps below 180°F the valve is open, with 90% of the oil bypassing the cooler. The remaining 10% of the oil flows through the cooler, maintaining constant system pressure, preventing air pockets and eliminating cold oil shock.