Two things are being blamed on the OCI; head gasket issues and oil consumption issues. Let's take a step back here and address these one at a time.
According to Ignatuis, the "acidity" attacks the "engine seals". First, what seals? Are we discussing the head gasket? It's a "seal", but oil has VERY LITTLE contact with the head gasket other than the peripheral circumference of the oil drain back holes from the head-to-block junction; the surface area of that contact patch is pathetically small. Not for one second do I believe that the OCI duration can affect a head gasket in that manner. Further, to what level of acidty are we talking here??? If you had some manner of tangible proof, some level of acid mangitude which causes the head gasket to decay, then I'd be all ears. If you could point to some level of acid, showing a slope or curve relationship to head gasket life, please share; it might make me believe. Some level of sound lab data or field study would be very helpful. Otherwise I don't buy this for a second.
As for seal life and oil consumption, I'll again ask for proof. It is not I should provide proof; I didn't make the claim. He who stakes the claim should provide the evidence to back up the claim.
- Why did Subaru shorten the OCIs? Per your claim it was to reduce oil consumption.
- where does the oil consumption come from? Is it blow-by past the rings? Is it valve stem seals erroding? Is it PCV escapement?
Unless we know the route of oil leaving, we can't really understand the rationale for the shorter OCIs, can we?
And how does a shorter OCI make for less oil consumption? If it's acid as you claim, why does Subaru not advise it's customers and dealer service centers to get TAN analysis on every oil change? I mean, how do they KNOW that 6k miles is safe, but 7.5k miles isn't safe? Different oils have differeing levels of TBN, so it's not likley they can say that any one OCI is too short or too long, can they???
Example: A car that sees lots of short trips in a humid area like the gulf coast is going to see more acid accumulate in the oil more than a car that lives in the arid desert and drives long cycles. Wouldn't the desert car running 8k miles likley have less acid than the gulf-coast car running 5k miles???
My point? The OCI duration alone cannot predict the level of acid in the sump. Any one specific vehicle can have more or less acid than another car, with the exact same OCI, because of drive cycles and environment. So to say that "acid eats the seals" is the reason Subaru shortened the OCI seems, well, perposterous to me.
Now, it could be possible that shorter OCIs make for less contamination left behind; less debis in the oil means less debris on the surfaces of the head. High deposits of sludge could cause "hot spots" where the oil can't cool the head well, and that in turn makes the head get too hot in localized areas, which in turn may cause the head to warp and/or the head bolts to stretch. But that goes back to my comments before regarding design and component material choices.
* After all, that was Toyota's answer many years ago when they had engines that sludged up; shorten the OCIs. Is that the root cause here? Long OCIs are causing the Subie heads to get sludged up and too hot and warp?
* Saturn SL2 engines had piston rings with no oil drain back, and so they had rings coke up when longer OCIs were present; they changed the OCIs from 5k to 3k, as I recall. Of course, that really didn't take into account whether you were using a low group II oil or a premium PAO or GTL (which would resist coking to a higher degree).
Ya know what would help here? A TSB reference or some credible data showing actual causation and not conjecture based on correlation.
I have an engineering degree. I worked for Ford for 16 years. Then I transitioned to HVAC manufacturing. I did quality control and statistical process eanalysis for a living for over a decade. I ran all manner of tests in HALTs and field studies. I helped write DOEs (design of experiments). I've studied oil analysis for over a decade. In no way am I an "expert" as to knowing what Subaru chooses to do, but I can understand sound logic when I hear it, and I recognize a desparate attempt to deflect legitimate questions when I hear it as well. Nothing you've written here so far convinces me that acid causes head gasket problems; in my mind that is next to impossible. And while I might agree that sludge could be a cause for shorter OCIs, I've not seen any direct proof that's why Subaru took this path as part of their rationale.
To use your own example as proof of my complaint ...
Smoking does not cause cancer. Carinogens cause cancer. Smoke is in correlation to cancer, but carcinogens cause cancer. THAT is a distinction which seems to elude you. Smoking cigarettes which have been heavily manipulated by the OEMs to contain all manner of addictive chemicals, many of which are proven to be carcinogens, will have a high propensity in some people to develope cancer. Some people who smoke those same cigarettes don't get cancer because of genetic resistence to those carcinogens; not everyone who smokes will get cancer. Further, not all cancer is caused by smoking; other things contain carcinogens as well. Smoking cigars has been shown to have a much lower risk of cancer relative to cigarettes, though it is still elevated over not smoking at all. I realize it's the easy path to simply say "smoking causes cancer", but the reality is that's not true. Smoking cigarettes has a higher probability of resulting cancer in some folks because of many factors including genetics, carcinogens, frequency, age, other things like acohol consumption, lifestyle choices, work environments, etc.
Smoking is in correlation with cancer. It may result in cancer in some people, but it does not cause cancer in anyone.
Carcinogens are proven to cause cancer; that's the defintion of what they are.
So please go find me proof that shorter OCIs help head gaskets last longer in Subarus, or prove that longer OCIs are the "carcinogen" of head gasket and seal life.
To be clear, I'm not saying you're wrong.
I'm saying you've not proven you're right, and many of the claims you've made are improbable at best to me.
Simply put, what is it in a longer OCI that causes head gasket failures or seal failures? And don't say "acid" because unless you can back that up with real study data, I call bovine manure on that basis.
the OP posted a UOA and u came in implying that his engine would suffer dire consequences due to extended OCIs even though the report was good/great.
