It would be worth organising some sort of wire lock to prevent the cap unscrewing, when not tightened hard down, I would have thought. Easy enough to do,and could save a lot of anguish.

Lodge Baker, don't worry, your Plus4 GDi is OK just as it is, with its' Ford designed cap, these are the Three Wheeler oilcaps that are causing alarm.

Paul
[At last, I have a car I can polish]