Check the valve cover gasket. It is very possible that your mechanic overtightened the valve cover and cut the gasket at the rear of the cover. Oil running down the back of a hot engine would likely pool around the bellhousing and not be too apparant on the engine itself. If he cut the gasket tell him to replace it and tighten it ONLY to spec. If he's used to American cars he's never had to deal with this before. If he is aware of the proper tightening proceedure then he may not have replaced the gasket. They frequently can be reused but when they get old they get hard and don't reseal properly.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold