The brief
The 420i had been leaving small oil puddles on the floor after parking, the cabin would catch a faint burning smell after a motorway run, and the low-oil warning had started flickering on the dash.
Those three add up to a slow leak that was getting worse. The burning smell is oil reaching a hot part of the engine and cooking off, the puddle is what makes it to the floor, and the flickering light is the level finally dropping far enough to register. On this engine the usual source is the oil filter housing, the module that holds the filter and an oil-to-coolant cooler, sealed to the block by gaskets that harden with age.
The diagnosis
On the lift the leak traced cleanly to the oil filter housing. The gasket sealing it to the block had gone hard and was no longer holding oil at the join, and once the housing was off it was clear the module had done its time, the plastic body grimy and tired.
The rest of the engine was dry, no other gaskets weeping and nothing wrong at the cover bolt seats. So the job was contained to the housing module: rather than just slot a new gasket onto an old 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, complete with fresh gaskets and a new oil filter, and the bolts were torqued to spec in the right pattern.
Then 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 after the road test, the oil level holding steady, and no warning light coming back.
The 420i went home with the engine sealed properly again. For the owner that is the end of topping up between services and the end of that burning smell on the highway. And catching it at the gasket stage, before the level dropped far enough to do any harm, meant a tidy repair rather than a much bigger bill down the line.