Originally Posted By: pitzel
Originally Posted By: RI_RS4
you have a hypothesis, but no actual science to back it up.
Well the NOACK volatility test is predictive of how much an oil will vapourize when actually placed in service in an engine. Where do you suppose the components of motor oil that are 'lost' when oil is placed into service end up? Into the PCV system, and ultimately, deposited onto those exhaust valves.
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You claim that if the oil were left it that the deposition rate would decrease. What do you base this on? Thus far NOACK volatility. However, NOACK volatility is measured on new, uncontaminated oil, not oil that has been fuel, ethanol and water diluted and degraded.
A chemical reaction, per se, does not occur between molecules of motor oil, and molecules of contaminants in such motor oil. If you dissolve one quart gasoline in one quart motor oil, you can, through the process of distillation, seperate the two components into their original constituents, ie: you're left with one quart gasoline, and one quart motor oil. The only exception to this is when you have combustion of the actual components, but this does not occur at the pressures and temperatures present in the crankcase.
As for water, and ethanol, these substances don't exist in motor oil, at engine operating temperatures, long enough to cause any harm (such as emulsification).
So what you're left with, in PCV vapour, is a mixture of blowby gases, and volatilized motor oil. Blowby gases, by definition, are gaseous combustion byproducts that don't cause much of any deposition (just look at the exhaust on a modern car -- there's extraordinarily little in terms of particulate matter deposited on exhaust components these days!). So what are you left with? Volatilized motor oil.
We know that new motor oil is much more volatile than motor oil that has been in service. So much of the solution revolves around ensuring that the oil that is in service has the lowest volatility possible.
If one keeps changing their oil every few thousand miles, this is just ensuring that the oil that's in their crankcase is as volatile as it possibly can be, creating exactly the problems as described in this thread.
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As a matter of fact, some of the RS4 engines in the UK that have had severe valve deposit issues had their oil changed per the manufacturers recommendation, with manufacturer recommend oil.
Maybe the manufacturer's recommendations are overly aggressive on oil changes. As we've discussed here extensively recommended OCI's are often not based on good science, but are rather, based on marketing or non-scientific criteria, good or bad.
So now you're changing your stance to fit the facts. First, the OCI is too short. Now you're saying it could be wrong and is too long.
As for ethanol and water in the oil, it does not all burn off, just as the fuel does not all burn off. The volatilization rate is dependent on the vapor pressure of each constituent. On Audi engines, we typically see fuel at the 1.5 to 2.5% level in the oil. We see KF water running from almost nothing to 2500 ppm, which would be about right, depending on the proportion of ethanol in the fuel. Ethanol is not measured, but most likely tracks water. At any given time there is 1% to 3-4% engine oil fuel contaminants being recycled through the pvc system.
Aromatic hydrocarbons from the fuels do interact with the oil in the combustion chamber, and do cause degradation when the reaction products are washed down the cylinder walls.
I agree that low volatility is a good thing. European engines often specify 0W-40 oils, which have just about the worst case volatility. That was specifically why we formulated RLI Biosyn at the high side of the 5W40 scale.
The other part of the chemical equation that you are missing are reported in TEOST testing. Unfortunately, very few manufacturers report their numbers. It's not just the fact that oil volatize, but whether or not it leaves behind deposits.
