Higher is better to a point. This is where the pH starts out in a motor oil.
Base is the opposite of Acid. Base raises pH and Acid lowers pH.
For example, pH of 7 is neutral as in water(H2o). So it the case of motor oil, the more acidic the oil becomes, the lower the pH becomes. I guess that around a pH of <2, the oil is becoming too acidic for aluminum.
Not sure that I said all of that correctly. But from my lab experience, I know what I am trying to say.
Sort of in the right place, but not technically correct.
pH is a measure of acidity and alkalinity in an aqueous medium ("pH" literally means "-log10[H+]", or minus log base ten of the concentration of hydrogen ions).
Oil is not an aqueous medium so the concept of 'acid' and 'alkali' is somewhat different and can not be measured or expressed as pH. TBN is a measure of an oil's alkaline reserve, which is used to neutralise acids. High or low is not good or bad because it depends on a number of factors as to what TBN is appropriate. Diesel engine oils tend to have higher TBN as diesel engines generate more acidic species in combustion. Longer drain oils have higher TBNs simply to allow them to carry on for longer. TBN drops in use, so the higher your starting point, the longer you go until the TBN is depleted.
High TBN in a UOA isn't always a good thing - the TBN is there to do a job - neutralise acids. If the TBN stays high then it isn't doing anything, meaning either there are no acids to mop up or that the acids are present but not being mopped up by the TBN. This idea of TBN retention is all very well, so long as the TBN is actually doing what its there for.