The spring ensures a constant leak-tightness around the shaft. Oil loss at the spring seal lip can be due to the shaft not being completely round in which case the crankshaft can be reworked with a shaft sleeve. Oil loss at spring seal lip can also occur due to excessive oil pressure in the crankcase which the spring cannot with stand. Finally any scoring of the shaft around the oil seal running surface can also result in oil leakage.
Oil loss at the outer edge usually occurs due to incorrect fitting (damaged to outer edge) or general wear and tear. Dirt and high heat can result in premature wear. When replacing faulty oil radial seals, lightly coat outer and inner edge of the seal with oil to ensure that no damage occurs when installing them.