The brief
The 520i had developed too much bounce over rougher patches, the nose was diving more than it should under firm braking, and the owner had spotted oil seeping out of one of the front struts. Three symptoms all pointing the same way.
The front struts combine the shock absorber and the spring perch into one unit. The damper inside controls how the spring moves: it pushes oil through small valves to keep the body settled. When the seal weeps, the damper loses the oil it needs, and the front end starts bouncing and pitching, more float over bumps, more dive under brakes.
The diagnosis
On the lift the driver-side front strut showed clear oil weeping past its seal, which is the definitive sign it has had its day. A bounce-test on the other side showed it was no better, the rebound damping faded well below spec, just dry rather than wet.
With one strut wet and the other tired, replacing only the leaking side would have left the front damped unevenly, one corner controlled and one corner soft. So it was a pair job, both front struts renewed together, with fresh top mounts and bump stops while everything was apart, so the front works symmetrically.
The work
Both front strut assemblies came out, and a matched pair of new BMW-spec struts went in with fresh top mounts and bump stops, the upper and lower mounts torqued to manual values.
Then the car went onto the rack for a settle cycle and a full four-wheel alignment, so the front geometry was confirmed square before it went back to the owner.
The outcome
No bounce over bumps, no nose dive under braking, and no oil down the strut.
The 520i went home with the front end planted and composed again rather than soft and floaty. For the owner that is a steadier, more confident car, and doing the pair at once means both front corners are matched and good for years, rather than a return visit when the second one finally lets go.