Same issue here. I drive mine on the hwy at 65-70 mph most of the time to shows 200 miles away. It will use a quart of oil every 600-700 miles and it still runs good. Might have found a slight crack in my intake gasket, so I am replacing it. It turns 3,000 rpm at those speeds and I do go up and down grades with it. It has a lopey cam and lower compression pistons. I think these motors use more oil than what we sometimes notice, depending on use.
What oil are you using? how is your compression? Mine checked out within 10-15% of each other, so no ring issues. How do the plugs work. Lol, you probably already know all of these things, but I am just spouting out ideas.