If you check Amazon customer reviews for this product you will find that many people have had permanent success at stopping oil leaks with this product.

 

 

My valve cover is also leaking on my '95 XJR. I replaced the outer seal and plug well seals a few years ago now, but I did not replace the seals around the bolts. This may have been a mistake as the old bolt seals may have compressed reducing the clamping force on the seals due to the shouldered bolts mentioned above. At the time I cleaned out the oil separator plate / mesh in the cam cover with petrol before refitting. I probably need to look at my hose too.