I had trouble inserting my key into the lock. It always seemed that the last 2mm did not ho in. Moving side to side and up-down made it work but it was annoying. I tried cleaning the internals of the lock (WD40, brake cleaner etc) and greasing with Teflon PTFE grease spray. It helped but just a little.
When at some point I had trouble opening the passenger side door I had enough. I decided to find out what the problem was.
Turned out that the key wears. Especially the front but was more shallow that it should be, thinner in thickness so to speak.
I tried my spare key that was at least .5mm thicker and worked better. Not perfect either but better.
So I decided to take my soldering iron and see if I can add a little layer on top of the metal. That worked just fine. Normal PbSn alloy used for soldering pcb and components. But the alloy is rather soft so I wiped it and used a small layer of silver soldering. This is harder. Tried the key and it works perfectly again.
Best solution is purchase a new HUF key blade for the 928. But 81 USD was not what I had in mind.
1992 928 GTS Midnight Blue