The original oil filer cap is made from plastic.
In an attempt making sure only I fill things into the engine, I bought the Wunderlich oil filler cap. I paid Euro 45 / US$ 65 for it.
The cap has a free spinning top. It comes with a supplied tool for locking the top, so that the cap can be turned and removed. Well if you have this tool, then its easy to find another item which can replace it anywhere. But at least trespassers would not be tempted performing an unwanted top up, with something that possibly is not even oil.
To be honest I bought it mainly for the looks. The plastic cap looks cheap, but also is not significantly easier to open. Hmm. What is possibly a bigger disadvantage of the original cap: The tool needed for opening is bulky, while the Wunderlich system allows for a small key.
