Why do people use time as a factor for OCIs?

[martyi]

Everywhere I look people say things like they change oil every 3000 miles or 3 months; 5000 miles or 6 months; and in some synthetic oil applications, 10000 miles or 1 year. Even my Honda owner's manual says to change oil when the OLM tells you service is due or once a year, whichever comes first.

Is it not usage of the oil in that car that really counts as to its life expectancy? And usage only comes from driving miles. What does how long the oil is in the car have to do with anything? I mean it won't spoil or rot will it?

Guys at the lawn tractor forum talk about how you should change old oil after the season rather than in the spring. The lawn tractors then sit up for six months without being used but have new oil in the pans. I also would think oil has a very long shelf life just sitting in the bottle.

I know certain conditions such as towing, short trips with cold starts, dusty conditions or other severe use can shorten the OCI but these are all based on usage. What does the number of months have to do with it? Or is it just old habit and an easy way to keep track of oil change intervals, whether a change is actually needed or not? Have we not reached a point in time where useage (under our particular driving conditons) should be the only factor to consider in OCIs?

 

[Ugly3]

Well, if someone drove 3 blocks to work and back each day it would take many months to get the miles on the engine. Time, it seems to me, is just a safeguard against the worst case.

 

[mountravlr]

One syn blender uses the "X miles or 1 year" to allow for infrequently used vehicles, and the contamination issues which accompany long OCI's. They are "covered" with regard to the oil's warranty if someone goes the mileage limit, but exceeds the time limitation. This blender requires strict documentation of OCI's for warranty purposes.

 

[TallPaul]

Isn't it that there always is some amount of blowby past the rings that contaminates the oil? If one takes mostly long trips, this blowby is minimized by 1) fewer cold starts and 2) hot oil temp burning off much of the bad stuff. Short trips promote and maintain much of the blowby effects in the oil. So on a short tripper car, maybe 3 to 6 months, whereas on a long tripper (and given synthetic) one might be able to go a year.

My motorhome is sitting now with 11 month oil oil in it. It has has only long trips, but few of them and has sat for several months at time. But the oil looks good on the stick and on a blotter. And that is regular Maxlife (dino with about 15% PAO).

 

[Big Jim]

This was a bigger issue before fuel injection.

 

[Audi Junkie]

I do time intervals as a convenience. For instance, when getting a state inspection, it's convenient to get an OC too. Seasonal factors play a role as well. ie: thinner for winter / thicker for summer. It's also nice to do a change before it gets super-cold outside, and then when it gets warm again.

 

[mountravlr]

Acid contaminants are dealt with by the oil's TBN, a part of the additive package, among other factors. When the TBN drops to around 1, it's time to to change oil, regardless of mileage/time considerations.

 

[labman]

My cars sort of live between the severe and normal usage. I have gone to the 3 month. With the job my wife had last year, some of the quarters, the car hit 7K. Other times, with fewer long trips, it barely hits 3K. When I used to do 6 month ones, I ran into a big sludge problem.

 

[LarryL]

You could also consider using gallons of gas burned as an indicator for the oci. Any kind of driving situationthat that causes poor gas mileage also could cause more wear and tear on the engine and the oil. The problem is that using gas consumption means that there is one more thing to keep track of and most drivers don't care one way or the other. A friend has some service trucks that spend their life in the California desert and he uses gas consumption for the oci with good results. The driving is a mix of high speed driving and very slow speeds in four wheel drive. Each truck ends up driving a different mix of miles, but using gas consumption gives him uoa's that are very consistant.

 

[pitzel]

I think one explanation is found within the interpretation of the GAAP rules of accounting that more or less require that companies 'quantify' the extent of the liability they face under warrantys or guarantees.

For example, when Mobil sells a quart of their "1 year/15k" motor oil, they know that, from an accounting standpoint, they are completely absolved of possible claims 1 year after they sell the product. They can then accurately account for any potential liability relating to customer equipment damage on their books, and there is little uncertainty as to the duration of any potential liability or claims.

Whereas, if Mobil specified 'unlimited guarantee, use UOA', some people could, in theory, run the stuff for 5-10 years, with UOA. But XOM would have to keep an reserve on their balance sheet relating to warranty claims indefinitely.

... Of course, they all want to sell more product, so that is one of the motivations as well.

 

[cyclic]

You should really go off engine hours, that is the only accurate way to measure how long the oil is in use.

Now for fleet services I know we use 200hrs on dino oil as a measure for the max time. Now when I drive my commute which is 80% hwy, 200 hrs represents 8000kms roughly so there ya go, 200hrs is a good measure of time, I'd say 300 on synthetic.

This is why the new Crown Vics have engine hour meters in them, great idea.

 

[Shannow]

I had 10 months on the M1 in my 4Runner, with only about 3k miles, a large number of them occurring in groups of 2 or 3 (better half with two young children and crappy weather).

The blotter test showed that the oil was knackered.

 

[TallPaul]

quote:

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Originally posted by Shannow:
The blotter test showed that the oil was knackered.

---

If it's not too difficult, it would be interesting to see that blotter.

 

[Shannow]

hmmm...will work on something.

The centre, where the initial drop was stayed very dark (not sticky or raised), and although the oil spread, it didn't take much if any of the centre with it.

my turbodiesel with three times the mileage, but a little over half the time spread similarly, but dragged the soot evenly.

 

[TallPaul]

I often get rings that darken through the oci, but the only time I got a black center was with AutoRx.

 

[jagabom]

I always change my oil in my lawn mower in the late fall. You change it and run it for a few minutes. this gets all the **** out of the engine and leaves it in the crank case for the winter. As for changing it in the spring after changing it in the fall, I don't know.
Jim
with a lawn mower.

 

[TomJones76]

Mileage as an OCI controller presupposes failure due to "usage".
Time as an OCI controller presupposes the possibility of failure due to oxidization.
If your engine or driving habits are less than ideal, you can wind up with gas or water in your oil.
Once you've got gas or water in the oil, your oil will start oxidizing.. maybe a little bit, maybe more, maybe less. Saying "3 months" or "1 year" protects you from the risk of this oxidization killing the oil before mileage would cause you to hit your mileage-based change interval.
That being said, 3 months is probably conservative for any car except one that gets used for something really abusive, like 2 mile trips to work and no other trips.
6 months is probably a better conservative guideline based on the OCIs we've seen, and most oils will take you a year under most any circumstances.
That being said, if I was in the business of providing warranty service on new car engines, I might think that 3 months was a pretty good guideline.
If I was Jiffy Lube, I'd probably think 2 months was an even better idea.