The brief
The X3 had been leaving oil spots under it, the owner could smell burning oil after longer drives, and a look in the engine bay showed oil seeping around the valve cover with some of it tracking down toward the spark plug wells. He brought it in before it cost a coil or worse. The valve cover sits on top of the engine and seals the camshafts and valvetrain in. On these it is a plastic cover, and over years of heat cycles the plastic warps and the gasket hardens, so oil starts weeping out along the seam. That is the spots on the ground and the burning smell when it drips onto something hot. If it tracks into a plug well it soaks the coil boot, which is where misfires and a check engine light come in. All of it traces back to that cover.
The diagnosis
On the lift the leak was confirmed at the valve cover. The plastic had warped enough that the gasket couldn't seal along part of the seam, and oil had been weeping down the side of the engine, with one plug well showing oil in it. The other gaskets in the area were dry. That's a valve cover replacement, not just a re-gasket. A warped plastic cover won't seal reliably no matter how good the new gasket is, so the cover and its gasket both get changed, and the oily plug well cleaned out while it's open.
The work
The ignition coils and the old valve cover came off, the camshaft area and the oil-soaked plug well were cleaned up, and a new genuine BMW-spec valve cover went on with a fresh gasket, the bolts torqued to spec in the proper pattern. The coil boots were cleaned and everything reseated, and any stored fault codes cleared. Then the engine was run to confirm it was firing cleanly with no leak, followed by a road test.
The outcome
Dry valve cover, a clean bay liner after the road test, no burning smell, and the engine running smooth with no fault codes. The X3 went home with the top of the engine sealed properly. An oil leak at the valve cover only spreads, and once it's into the plug wells it starts costing coils, so sorting the cover when it did kept the job to the cover and a clean-up.