As a test consider putting in straight 30 or 40 weight diesel oil on the next oil change. If consumption drops way off the oil control rings are weak. Oil usage should go to about 1/2 the present consumption. If if there is something wrong with them.
Its warm enough to run the heavier oil this time of year. It becomes really interesting if oil usage were to remain identical. Just adding the straight oil every 100 miles or so would also start to show if there is any difference over time as well. Just not as defined.