An oil seal, also known as a shaft seal, is a component used to prevent the leakage of fluids from within engine or transmission. The oil seal is typically made of rubber or other flexible materials and is placed around the shaft of a rotating component to prevent any fluid from leaking out. This can include engine oil, transmission fluid, or other lubricants that are vital for the proper functioning of the vehicle. Without an effective oil seal in place, these fluids can leak out and cause damage to various parts of the truck's system over time.