You are asking too general a question, it could be a Head Gasket, timing chain cover or valve covers, clean the engine and run it till it leaks and find where the oil is leaking from

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Jerry

2004 SL500 Black 55K Miles
2002 s500 Black 90K Miles
1986 300E Gold 233K Miles
1966 Twin Cessna 320D
1989 Jeep Cherokee 233K Miles
1994 Ford Van E150 399K Miles