I had same problem on my 99 4.6. I chg'd to 5w30 mobil 1 at 18k miles and immediately noticed
increased oil usage to the tune of amost 16oz in 1600 miles at intersate speeds. Much less in town. I talked to the dealer about an oil consumption test but I had to set it up for high speed driving to prove the usage. Anyway it startewd using less and less and at about 50k miles it quit using oil. Don't panic....