It is a good read and has lots of easy to understand concepts that make up modern oil change computer decisions.
SO HOW OFTEN SHOULD I CHANGE MY OIL?
You can never change your engine oil too frequently. The more you do it, the longer the engine will last. The whole debate about exactly when to change the oil is somewhat of a grey area. Manufacturers typically indicate every 10,000 miles or so. Your mate with a classic car says every 3,000 miles. Ole' Bob with the bad breath who drives a truck says he's never once changed the oil in his ve-hicle. Fact is, large quantities of water are produced by the normal combustion process and, depending on engine wear, some of it gets into the crank case. A good crank case breathing system ensures the water gets removed from there PDFQ, but even so, in cold weather a lot of condensation will take place. This is bad enough in itself, since water is not noted for its lubrication qualities in an engine, but even worse, that water dissolves any nitrates formed during the combustion process. If our memory of chemistry serves us right, that leaves a mixture of Nitric (HNO3) and Nitrous (HNO2) acid circulating round the engine! So not only does it suffer a high rate of wear at start-up and when the engine is cold, it suffers a high rate of subsequent corrosion during normal running or even when stationary.
The point we're trying to make is that the optimum time for changing oil ought to be related to a number of factors, of which distance travelled is probably one of the least important in most cases. Here is my selection in rough order of importance:
- Number of cold starts (more condensation in a cold engine)
- Ambient temperature (how long before warm enough to stop serious condensation)
- Effectiveness of crank case scavenging (more of that anon)
- State of wear of the engine (piston blow-by multiplies the problem)
- Accuracy of carburation during warm-up period (extra gook produced)
- Distance travelled (well, lets get that one out of the way)
If very clever (or obsessive) owner could probably come up with a really clever formula incorporating all those factors. However, we would give 1, 2, and 3 equal top weighting. Items 1 to 3 have to be taken together since a given number of "cold" starts in the Dakar in summer is not the same as an equal number conducted in Fargo in January. The effect in either case will be modified by how much gas gets past the pistons. What we are really after is the severity and duration of the initial condensation period. All other things being equal, that will indicate how much condensate will be produced and we would suggest that more than anything else determines when the oil should be dumped.
DAMMIT - GET TO THE POINT ALREADY!
Hang on a moment - if you really want the answer, there's a couple more factors that need to taken into account: Crank-case scavenging (that's the clever term for sucking the nasty fumes back out of the crank-case) - or lack of it - is a crucial multiplying factor affecting all the other items listed above. As an example, the worst we've heard of was a Ford Fiesta of the mid 70s or so. Its crank-case fume extraction was via a tiny orifice directly into the inlet manifold which obviously could not handle any significant volume of crank-case fumes without upsetting the carburation. The car in question had been used almost exclusively for 5 mile journeys to/from work, shopping etc, and it had always been serviced "by the book". Despite (or because of) this, the engine was totally buggered at 40,000 miles. Alternatively its possible to find a car that by virtue of excellent crank case fume scavenging could tolerate many more cold starts than one without.
Taking all these into consideration, our philosophy would be to totally ignore the distance and change the oil twice a year - about November and March. Move these dates a bit according to the severity of the winter. An average family car will do around 14,000 miles per year and about 2/3 of that will fall in the March - November period. At the end of that period, the car will be getting close to the manufacturer-recommended oil change interval - but all that distance will have been done at reasonable temperatures, including long distance runs during vacations and good weather. During the November to March period it may accumulate only two or three thousand miles, all low temperature starts and mostly short runs.
Around 1995, an article in the ANWB journal (ANWB is the Dutch equivalent of the AA - or the AAA in the American case) reached more or less the same conclusion that distance was not very important. In their case they applied this to their road service fleet, which once started in the morning never got cold. In effect, they hardly ever changed the oil. It worked out at something like 30,000 miles between oil changes. They also had some kind of water or acid indicator attached to the end of the dipstick and went by that rather than distance.
THAT'S A POLITICIAN'S ANSWER - TALK ABOUT DODGING THE ENTIRE ISSUE!
Dodging the issue? We don't know how far a car is driven in a year, where it lives, the driving style of the owner, and many other things, so we can't tell what's right for any particular car. This author changed the oil and filter in his 1985 Audi Coupe every 5,000 miles. It had done over 150,000 miles when it was finally sold, wasn't leaking and didn't consume any oil. My Subarus got oil changes at 10,000 miles but were newer cars in a warmer environment. My VWs got oil changes at 8000 miles or so. If you must have a figure, then 8,000 is it.
MAINTENANCE MINDERS - WHEN THE CAR TELLS YOU WHEN TO CHANGE THE OIL
A lot of cars now come with maintenance minders - inbuilt systems designed to tell the driver when to change the oil rather than leaving it to guesswork. First generation systems were nothing more than mileage counters. When the car reached an interval of 3000 miles, the light came on. Now they're more involved. The system typically monitors driving style (in terms of how long is the throttle open for any given duration of driving), air intake and external temperatures, coolant temperatures and variations and engine timing (determined by load on the engine and octane of fuel). There's a clever formula that each manufacturer keeps to themselves that can take in any or all of these factors (and probably more) to determine how fast an average oil will be ageing in the engine. When it gets to the point where the system decides that time is up, a light will come on on the dash - typically "Maint Reqd" or "Service Reqd" (Note - this is NOT the "check engine" light). On some cars the various trip computer settings can be cycled through to see an oil life remaining readout - normally given as a percentage. This is a convenience function so the driver can tell at any point in time roughly how much longer to before an oil change. That's useful for planning road trips - don't go on a 2000km trip if the oil life remaining indicator shows 10%. On cars with these systems, I would defer to the onboard computer rather than trying to work it out for yourself. When the system tells you to change the oil, just do it. I've found that my 8,000 mile guesstimate above is pretty close to when the maintenance minder in my cars has indicated an oil change was due.