300se oil consumption

[SCOTTFISH]

Has anyone ever had a problem like with my 89 300se where the car does not use much oil in city driving, but take a trip of 500 miles or greater and it will consume about a quart and a half during the trip. I have had a valve job done and the problem got worse. I took the car back and they reworked the heads and it is back to where it was before the valve job was done and they say that everything is the way it should be. I suspect the rings are gummed up and the local dealer suggested switching to Mobile 1 synthetic 0w 40 oil. They stated that it would clean the rings up some and if they are just gummed up, would help with the oil consumption. Help? Scott