 partially plugged up and you're on the highway so you're you know 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 a you know the internal cleanliness of the engine type thing we have 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