Hello, all, I've been an enthusiastic learner here for a while, and I know that a lot of variables affect one's engine and choice of OCI. I'd like to extend mine a little. Here's the deal: Car: '97 MB C230, normally-aspirated 4-cyl.; does not require synthetic oil. Commute: about 9 miles to work. Approximately 5 of those miles are done at 50-60 mph; the rest at 35-40, with a few stoplights. I do the same 9 miles back home, but most of the 5-mile 55 mph run is bumper-to-bumper traffic, with speed averaging about 20-25 mph. On the weekends, I do a lot of short-trip driving, groceries, car wash, etc. I try to get in a 10-15 mile freeway drive two or three times a month. My total mileage for a month averages about 800-900. (There's not many places to go here in The Swamp.) Right now I'm doing 4 months/3200 to 3600 miles between changes. Oil is usually Delo 15W-40 thinned with a little 5W-30 Havoline; now and then, TropArtic 10W-30 syn blend. I realize modern dinos can go longer, but with my short-trip setup and near-severe service, I'm uncertain. Would five months/4000 to 4500 miles be OK? Generally, what would be a reasonable OCI? -- Paul W.