The Oil Change Question


Just read the latest column. Someone calculated that their manual’s recommendation of an oil change would, by their calculations, mean a change every 83 hours at an average 60 mph. The questioner had installed a run time meter to track when oil changes were needed. The brothers, in their zeal to impress one another with their stellar wit, engaged in a convoluted line of questionable reasoning and recommended every 100 hours.

Have they never read the terms of a warranty? I’ve never seen one that gave you the option of using hours instead of miles.

And besides, the manufacturer recommends every 5,000 miles…why would one recommend using hours unless the vehcle were being used in some extremely unusual fashion like spending much of its life idling (like a semi or cop car would).

This post has been moved to the new Car Talk Discussion Area, by a Car Talk Lackey. The original poster is Mountainbike.


Remember the part about miles or months? You’re probably covered any way you do it. Just make sure to be within warranty. The car already has an odometer at no extra charge. So, if you do it as cheaply as possible…


100 hours at 60mph is… 6,000 miles.
83 hours at 60mph is… 4,980 miles.

See where the numbers are coming from?
The hours are simply an equivalent for the mileage interval.
The difference in the hours interval suggested is merely a difference in opinion.



And my question is how much can one add to the interval if using synthetic oil? My 2002 Prius had a 7500 mile interval, my 2005 Prius has a 5000 mile interval. I suspect it got lowered because of the “waxing” problem, which synthetic does not have. Any comments?


Not really. Because 100hrs x 20mph = 2000 miles, which might be typical of urban traffic. If you drove exclusively in an urban environment and changed the oil every 6000 miles, you would changing every 300hrs of operation. The idea is that it is hours of operation that is the true indicator of wear and not the necessarily the distance. Airplane service intervals are usually expressed in hours of operation.


Actually the person who mentioned urban driving is close to a good reason to use hours: urban driving is harder than suburban/country driving with it’s stop and go. That’s the only reason to me to use hours but I’m going to continue to use Mobil 1 (sure it may be overkill but it’s extra insurance) and the odometer for my oil changes.


Yes, airplane service intervals are generally handled based upon hours, however this is misleading.

For most piston aircraft, it is not done on the basis of clock hours, but rather on the basis of tachometer hours.

The tachometer clock is calibrated such that at some specific rpm 1 clock hour equals 1 tach hour. However, lower rpm will make 1 clock hour equal less than 1 tach hour, and higher rpm will make 1 clock hour equal greater than 1 tach hour.


Probably the best method would actually be “fuel used.” I worked at one large trucking company that switched to that method. They found that the amount of fuel used was a much better indicator of how much “work” the engine had done. On a team truck that sat very little and did mostly long runs, the fuel threshold (which at that time was 3100 gallons) brought them due for an oil change at about 21,000 miles. A solo truck, which would do more city work, idle more, etc., might be as little as 12,000 miles. Their oil testing procedures confirmed that this method was effective, and the manufacturers approved it.

Just as an aside, changing oil too often is a waste of valuable resources. In a trucking company with 10,000 trucks and which buys oil by the tanker truck load, it can make a huge difference. Especially since they are no longer allowed to filter it and add it to their fuel.


US 2001-2003 Prius had a 7500 mile/6 month interval, while US 2004-current Prius have a 5000 mile/6 month interval. UK Prius over the same timespan have a 10,000 mile/1 year interval, as does much of the rest of the world…

My personal speculation on the US interval change is to have people rotating their tires more often (based on the mileage interval change listed in the supplemental tire warranty that was issued for edge-wearing 2001-2002 US Prius tires). Online, I’ve read speculation about a change in Toyota’s oil supplier, or a need for more changes due to the higher RPMs allowed on the newer Prius, or just Toyota USA standardizing their maintenance interval across the product range, or even speculation about a vast conspiracy to get you to be “serviced” more often… The real reason?