Hi Tom,
It is correct that most literature, if not all, list the total TOST time or time to reach 2.0 mg KOH/g. This requirement is appropriate/practical for Group 1 oils, but not for Group 2 oils, because of the difference in the thermal stability between these base oils (amount of aromatics present in them), and the authors of new books seems just keep rewriting it. A typical TOST (to reach 2.0mg KOH/g) for G1 oils is between 1500-3000, and for G2 oils is more than 10,000 hrs. Consequently, the TOST analysis for G2 oils is extremely time consuming and expensive to run. Also, G1 turbine oils when new have AN from 0.3-0.5, while G2 oils' AN is below 0.1 (usually in the range of 0.01-0.05). As more and more lubricants are formulated with G2 base oils, it was appropriate to modify the test by limiting its duration and lowering the targeted level of AN. It make sense to me for at least two reasons:
1. Economics (more affordable test to end-user, where they don’t wait several months for results)
2. Practicality (results are more useful, because they are closer to condemning limits for the oil. I consider it useless sticking to 2.0mg KOH/g, because it doesn’t tell much to the end-user, because nobody in their right mind would use their oil that is reaching nearly that level).
As for acid number vs. time graph that you would like to know, it would be best to ask a lab to do a battery of tests to determine the rate of AN changes. However, this may not be applicable to oils across the board, because of difference in the quality of base oils.
