Recently bought my daughter 1992 300E 4matic to drive to school. It has 107,000 miles and car seems to be in very good shape. When checking fluid levels today noticed oil in coolant overflow tank. Oil covered approximately 1/4 of tank. No moisture or water on dipstick. Is this normal practice to add oil additive to antifreeze for water pump lube or have I purchased a real problem. Thanks Rickjr.