Combustion gases are not the main source of acid buildup. They are not highly acidic in themselves. There are various causes in a normal engine, and their accumulated presence will cause your problems in a a parabolic way. One of the major causes of acid production within the crankcase is in the presence of moisture, both as a combustion byproduct and as sourced from atmospheric water vapour when the engine cools down after it is turned off. Water chemically attacks mineral oil and its derivative oils, breaking them into smaller chains which are more easily used in acid production. The acids then act in a more aggressive way towards the remaining oil molecules. Obviously base packages are designed to slow this process down as best as is possible. I run Redline 15W40 and a hydrophyllic bypass filter. At 103 000km, TBN was almost unmoved, indicating that the base additive had not been consumed. It is interesting that even a mineral oil used with a hydrophyllic filter will get greatly extended service life out of the oil. Because poly-ol-ester is highly resistant to water the whole acid issue is highly reduced. As a case in point, during my 100 000km, I picked up fuel which contained a fair amount of water. I hadn't drained my fuel/water separator in years and then in the space of several weeks I had to drain it several times. This showed up in the oil analysis at that time as a very high amount of water in the oil, and a highly increased viscosity (But not over the 22.5 mark). I then changed my bypass filter. In the subsequent analysis results, viscosity returned to normal trend levels. At 103 000km viscosity was at 19.2 centistrokes (at 40 degrees Celsius).
In my case, it would have been the level of pentane solids (soot) that would have required me to change it. The chemists suggest a basic rule of thumb of 50% increase in viscosity(at 40 deg). In mineral oils, it is the other factors that come into play long, long before soot (from combustion gases) is even looked at.
If your oil is not breaking down in acid production, it will be able to do its job of lubricating.
Bit of trivia:
Poly-ol-ester is highly water resistant. It is also highly bio-degradeable. Apparently soil borne bacteria find it pleasant to their palate.
