And this entails merely making sure ALL traces of old sealant be removed.

Then, thorough de-greasing before using the proper sealant.

Pretty danged easy. Total cost? ~ $12.

And, since no one else pointed this out:

I doubt seriously that it is the valve cover gasket(s) that are/is leaking. It will be one or both of the "Breather" covers on each valve cover. They're sealed with an RTV-type sealant.

Apparently this is one area of expertise the Americans have over the Germans! The OEM stuff they used is...crap.