There are some bolt on parts in that neighborhood that need a good look for possible oil leaks.. Timing chain tensioner, Cam position sensor, Vanos Solenoid and valve cover gasket slipped a bit in the re-install and forgetting to put gasket sealer in the half round neighborhood where it dips down at the end of the cams could all be possible.
Also., I talked to a race engine builder at one of our local auto parts stores, a place where they really know their stuff. He said to use block sealer in the coolant and drain and refill coolant after a couple hundred miles. He builds 10K rpm motors and knows his stuff.

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2001 x5 3.0 (sold with broken motor), 1990 e30 M3, 1991 318is, 2002 325i, 2008 335i Dinan stage 3 6 speed,