Hello, all, I'm just back from a trip to Knoxville, TN, and the Smokies. The big grey beast, the Buick, and I did about 1600 miles, a lot of highway (ca. 70 mph) and plenty of steady cruising at lower speeds, plus some good exercise for the car going up and down mountains in TN and hills in AL. The car performed like the thoroughbred it is. I had just changed the oil, to Castrol GTW 5W-30, and had 1000 miles on it before I left; now I have about 2600. The OLM dropped much more slowly, as I'd expect with lots of highway. Normally at 2600 miles I'd be 3 months into my OCI and be looking at 64% life left. However, it's now reporting 74%. Assuming I don't have any hurricane emergencies this summer, I'll probably be back on the usual 850 miles/month, and the oil life goes down by about 12.5% every month. Can I leave the oil in there until late September/early October, which I estimate will leave me at about 14% life left (a 6K OCI)? Or should I change it at 5K, around the end of August, which should have about 27% life left?