What type of sealant or gasket do you use on the rear differential cover a 2003 Ford F250 Super Duty truck?

For a 2003 Ford F250 Super Duty rear differential cover, you should use a high-quality RTV silicone gasket maker. Don't use a pre-made gasket. RTV silicone is specifically designed for this application and provides a superior seal, especially when dealing with oil and the vibrations experienced in a truck's differential.

Look for an RTV silicone that is specifically rated for automotive applications and oil resistance. Many brands offer this, such as Permatex, Fel-Pro, and others. The specific product name may vary, but look for descriptions like "Ultra Black RTV Silicone Gasket Maker," "High-Temperature RTV Silicone Sealant," or similar. Read the product label carefully to ensure it meets the requirements for differential applications.

Important Note: Always clean the mating surfaces of the differential cover and the differential housing thoroughly before applying the RTV silicone. Remove any old gasket material completely. A properly cleaned surface ensures a good seal.