My XF had the same problem. It was the breather covers not the valve covers. I used high temp
RTV to seal them. I put it on the underside surface after I cleaned with soap and alcohol. I used the original aluminum screws, but cleaned the threads of the screw and the mating valve cover. I barely tightened them and let dry overnight. no problems for over 2years. If you use new stell screws make sure they are the correct length, I did use one to replace a lost aluminum screw, but coated with silicon grease to make sure it would not seize up. The only tools needed is the proper size driver for the screw head and a small scraper to clean the old sealant off.