My 1991 6Speed BMW 850i is running on 6 cylinders/half power. Any suggestions?

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I do have some type of oil leak but at the moment I can not tell where its coming from, I changed air flow meters, I changed relays and today I'm going to check if fuel pumps are working correctly and try and determine where the oil is coming from. Areas that come in my mine are, leaky intake gasket,fuel pumps, fuel filters, clogged Cat, clogged fuel lines or clogged fuel tank. Please I am very anxious to hear from RepairPal. Thanks