Take samples. Get it analyzed. Go from there.
I get my TBNs done every other change to see how the additives are doing. I started at every 5k and have been slowly moving it up by 500 miles at a time to see how the oil is affected. So far, I'm on course to the 7500 mile oil changes recommended by factory but I also drive like an old grandma. Once my 60k is up, I'm gonna switch to costco oil and see how it does comparatively.
