Care to post of pic of this?
To understand the Oil fill cap condition - the only guess I can come up with is they usually rotate 1/4 turn, then lift off - uses tabs and a groove to seat and hold them. Thats for Ztec motors, but later ones for USA just twist off. There is also Diesel and Petrol versions which also change the layout of where things are, so it's important to post a pic so we can hopefully find someone here on the site that can recognize it and guide you.
Now, if the gasket they use to seal the cap to the cover is bad, you'd see oil stain from it leaking too.
Else, are you having a problem removing the engine cover, the one that is not the oil cap itself, but the cover for the top of the motor?