 partially plugged up and you're on the highway, so your higher rates of speed, it might be getting a little more oil built up in that cover if it's not draining back, so it's sucking it in through the vacuum into the engine. It might be the internal cleanliness of the engine type thing. We replace valve covers on lots of cars. Never had to do it on a Subaru for this problem, but we have on many Silverado trucks. And sometimes they'll tell me, it burns oil on the highway, but it doesn't burn oil when I drive it in town.