Skeleton oil seals—also known as rotary shaft seals, radial shaft seals, or simply oil seals—are among the most widely used sealing components in rotating equipment. From electric motors and gearboxes to automotive axles and industrial pumps, skeleton oil seals prevent lubricant leakage and block contaminants from entering bearing cavities. Despite their simple appearance, these seals are precision-engineered components available in many profiles, materials, and configurations. Selecting the right skeleton oil seal for a specific application requires a clear understanding of seal types, material properties, and operating conditions. This article provides a comprehensive guide to skeleton oil seals, covering their construction, common types, material selection, and key installation practices.