Originally Posted by Wallowa
Yup carbonic acid is indeed nasty stuff that you do not want near any bearings.
Carbonic acid is a weakly buffered acid with a pH of ~6.0.
The acid that does the damage is H2SO4, sulfuric acid, a strong acid with a pH near 3. The H2SO4 comes from the combustion of fuels containing sulfur.
Quality motor oils with current API ratings contain calcium and sodium and other components that have opposing buffer capacity and neutralize the acids. Currently, engines fueled with diesel are mandated to use ULSD for on-road use, reducing the need for high TBN engine oils.
So, even your newer HDEO is lower in TBN than a few years ago. An engine oil with a TBN near 8 or 9 is desirable and has adequate capacity to neutralize acids formed by the combustion of even ultra low sulfur diesel fuel. Gasoline engine have even lower requirements for TBN so using a HDEO in a gasoline fueled piston engine provides increased protection from acid and poor quality fuels.