I'm thinking Mobil doesn't consider Amsoil competion just like Honda or Toyota doesn't consider Rolls Royce or Ferrari competion. Mobil sells on the shelves next to Pennzoil, Quaker State, Castrol, etc....

 

The leaks that resemble a cam plug leak on the 3.0 are normally the back of the valve cover gasket. The leaks start there and eventually spread to the rest of the valve gasket especially using synth since the molecules are round and more prone to slipping through seals. When you pull the valve covers to replace the gaskets you will notice that the gaskets are hard as a rock and brittle. Half the bolts will be loose also since the when the gaskets harden they shrink allowing play between the cover and the heads. These leaks are mistaken for main seal leaks since the it starts way in the back and weeps down to the bell. The fix is pull the intake, replace both gaskets. While there you will replace PCV, grommet, pcv hose and intake gasket. It takes a day to do the job. Don't forget to dab black RTV on the 4 corners of the head, valve cover, specified in the manual.