There are a variety of oil seals used in a variety of applications to help close the space between a stationary and a moving part.  By closing the space it prevents any lubricants from escaping.  Additionally, oil seals help create a seal that prevents any contaminants from entering machinery which can cause a host of problems.  Mechanical equipment and machinery require different size or type oil seals to ensure that there is a superior and precise seal.  Because oil seals help reduce contamination or mixing of materials it prolongs the life of machinery and helps ensure a more reliable performance.