To be honest, yes oil becomes "diluted" over time. But let us be more specific about that "dilution", and the time surrounding it.
Sure, if it's 20 years old and been sitting on the shelf, the additive package will be way out of date, and probably have seperated in the bottle and settled. This is an extreme that most all of us will agree doesn't warrant even putting the stuff in the crackcase. Or, if you've stored a vehicle that long, you should change it for sure.
What I'm talking about in regard to this thread is stuff that is less than 5 years old. Oil has no real idea how old it is. If you do an OCI, it's so much more dependent upon your use and severity, than the calendar.
I have a 1966 Mustang I drive around a bit in the summers. I run Rotella 10w30 HDEO in it, and OCI every 3 years. I only put about 100 - 300 miles on it a year; that's a very small amount of mileage. But when I do drive it, it gets good and warmed up for up to an hour at a time, so condensation is of little concern. The HDEO has a robust add-pack. Why change oil every 6 months, or even every year? The oil in my application simply cannot be "worn out" in that short useage. The oil has no idea how many days have passed since it left the bottle and entered the crankcase.
There was a UOA on here some time back where a guy ran synthetic for 4 years and 10k miles in a Toyota truck. The UOA came back fine.
Oil experiences a few things that make it worthy of changing.
1) contamination past a point of the host oil's ability to control (combustion byproducts, coolant/water, dirt are the main issues)
2) degredation of the additive package to a point where it cannot adequately protect the engine (detergetns, dispersents, TBN)
3) thermal breakdown (insolubles and sludging)
Until these things overwhelm your oil, it's not time to change yet.
There are three ways to determine your OCI. OCIs are most accurately predeicted by UOAs. In lieu of UOAs, then mileage is a reasonable alternative. Time exposure is the last option. Of the three, time is of the least consideration. Like I said, I would not go more than 5 years, but worrying about +/- 1 year is not worth the effort.
It's a matter of balancing your plan with your needs. My wife drives a large multitude of short 5 mile trips; she averages around 5k miles every 6 months, so that's my OCI plan. My new 2010 Fusion calls for OCIs every 7.5k miles or 6 months. I'm going with the 7.5k miles, even if it goes a bit over the 6 months. Am I to believe that I can do 7.5k miles in 5 or 6 months, but not 7 or 8 months? Did the oil suddenly turn sour and curdle after the 180th day? That's simply ridiculous! What if I only drove 5k miles in an entire year, but those miles were good heat-soaked miles that only happened once a week? Am I to believe that the additive package is going to just disappear in the 7th month? UOA evidence shows this simply is not the case. I feel perfectly comfortable running up to 3 years on an OCI, depending upon use and severity, with no concern whatsoever. I would not do it blindly, but I am willing to do it on a discretionary basis.
OCIs are an emotional choice for some, and a data-driven decision for others. We can't tell you how to pick; only you can do that. If you're bored and want to change oil, then go for it. But I don't believe oil has any real idea of how old it is. It does know how "degraded" it is, and the only way to discover that is with a UOA, not the calendar.
What I WILL grant you is that there are times when an OCI (especially with dino oil) is simply way cheaper than a UOA. In some regard, it's simply a more prudent decision to OCI because a UOA costs more. In this case, the assurance of the OCI outweighs knowledge of the UOA. For example, I don't ever UOA my Mustang oil; I simply change it. But I certainly do not OCI it every 100 miles, which is what would be about every 6 months! It gets the OCI every 3 years. I could probably go even further, but that's my personal limit.
The decay rate of the oil's ability to perform all its tasks is not so much a function of chronological age as much as it is one of exposure (mileage). And even that mileage has to be viewed with great scrutiny. I tailor my OCI and lubricant choices based upon a combination of:
operating conditions
environmental conditions
severity conditions
lubricant price point
ease of lubricant acquisition
desired preventative maintenace plan
etc.
"Agree, and also can't comprehend: Those fools buy a $40+K car, then ruin it by refusing to change $15 worth of oil."
Perhaps the reason some "fools" can afford a $40k vehicle is because they are not wasteful in their decisions?
