Cylinder lock "oiling"
Over the years, cylinder locks - e.g. the front door - always more difficult.
Before you run the risk of breaking the key when unlocking, you should rather ensure smooth running again.
In no case should sewing machine or other oil be used - this is just a huge mess, the key you would not like to put in your pocket :-)
Special lubricants are sauteuer.
I have had very good experiences with simple door lock de-icer, which you buy every winter and then let it rot in the glove compartment of the car.
The tips of the small tubes ideally fit into every cylinder lock. One or two splashes of the stuff in the lock and the key will turn over for the next few years as if by itself.
Important: After "oiling", close a few times and then wipe the key. After a day or two, there is nothing left of the admittedly not particularly pleasant smell, but the castle is still easy going :-)