One of our cars is “regular duty”. It’s used for commuting. The commute is (approx.) a 2 mile run through neighborhoods( a couple of stop signs, one light that is normally rolled through, then a 15 mile interstate highway run, then about 2 more miles of stop and go “city” driving. It’s very rare that it’s started for a trip less than 15 miles, and I’d rather take side streets at 20 mph then sit in a traffic jam.
The other vehicle falls under severe. It gets started, run 6 miles through neighborhood streets, never over 25 mph speed limit, shut off to drop the kid at daycare, then started and run 1/4 mile to work. This is 3-4 days a week; the other days are often short trips (2 miles to store, 6 miles to grandma’s house). This is also the Rental RV, so it gets loaded up and driven on the highway occasionally. 70-75 MPH, loaded down, AC blasting (small 2.5 liter engine).
My theory is that you need to run the engine long enough to get up to full oil temp – at least half an hour – every week or so to be sure that you’re getting the water out of it. If you do that than your driving is normal, bar towing or racing. In that case I change my oil about every 10,000 to 15,000 Km. I use Mobil 1 10w 30 in just about everything except the MGB, which gets an oil change every spring and uses dino 20w 50. I guess that I’ve never had a car long enough to wear out the engine (if you don’t count the aluminium block Vega that I had in high school), and I’ve never had a failure that could in any way be caused by long oil change intervals.
The race car gets a damn engine rebuild about every 20 hours – 1,500 miles – to magnaflux the crank and look for stupid things that happen to race engines. I get to look at bearings and journals that are perfect – they don’t even need to be polished, I usually put the same timing chain back in because the cam timing hasn’t budged. I don’t take the pistons out if the leakdown is ok. That’s on an engine that has 100 hp per litre, 12:1 compression and lives between 6k and 7k rpm’s which should be considered extreme duty.
My vote is that with today’s oil and filters, and even more importantly ring technology and combustion management, oil changes at 10k to 15k kilometers is normal and fine. Listening to oil manufacturers tell you to change three times more often is like asking the bartender if you should have another beer or asking the used car salesman if you should buy the undercoating and the extended warranty!
