The large one i put on the sump plug, but it was the wrong size and caused the plug to leak. The plug does have a seal in the head of the bolt, which is not possibly to replace. The only way i could stop the plug from leaking again was to renew it! It only cost a few quid from ford.
The mondeo does use 2 different sizes of sump plug (unsure why), so they give u an o ring for either size. Just bin them, refit the plug with the same seal, and if it leaks then get a new plug from ford :thumbup: