The brief
The 520i had been losing oil slowly, the owner finding fresh drips on the bay liner and traces around the oil filter housing. He brought it in to have the source confirmed and dealt with.
On this engine the oil filter housing is the usual suspect for a leak like this. It is the module that holds the oil filter and an oil-to-coolant cooler, sealed to the block by gaskets that go hard with age. Once they stop sealing, oil weeps out slowly, enough to drip onto the liner, and over time enough to drop the level far enough to set off a low-oil warning.
The diagnosis
On the lift the leak traced cleanly to the oil filter housing, the gasket hardened and no longer holding oil at the join. Once the housing was off it was clear the module itself had done its time, the casting oil-soaked and tired.
The rest of the engine was dry. So the job stayed contained to the housing module, and rather than slot a new gasket onto a part that was clearly past its best, the housing itself was renewed.
The work
The oil was drained down far enough to take the housing off safely, then both mating surfaces were scraped back to clean bare metal. A new BMW-spec oil filter housing module went on, with fresh gaskets and a new oil filter, and the bolts torqued to spec in the right pattern.
The engine was refilled with fresh oil and held through a warm-up cycle, watching the join for any sign of a weep before the car was road-tested.
The outcome
Dry housing, no drips, and the oil level holding steady.
The 520i went home with the engine sealed properly. For the owner that is the end of topping up between services, and catching it before the level dropped far enough to threaten oil pressure meant a tidy repair rather than the much bigger bill a starved engine eventually brings.