Oil is not Ok. It most likely is a sign of a leaking valve cover gasket. When I changed my plugs at 70K on my 4.4 the rear most plug on the drivers side had about 1/8 teaspoon of oil at the bottom. I had the valve cover gaskets changed and everything is dry now.

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2013 X5 3.5i M Sport
2004 M3 cabriolet SMG