Gaskets and oil seals should be replaced if worn or leaking, or whenever removed during servicing. Replacement is simple, but some engine dismantling may be necessary to reach them. Buy gasket sets from a dealer for the make of car, and state clearly for what parts they are needed.
Before installing the oil seal, it is important to lubricate it to reduce friction and prevent damage during installation. A silicone-based lubricant is recommended for most applications. If the oil seal is to be installed in a high-temperature or high-pressure environment, a more specialized lubricant may be required.