Hi macabf,
Please note that I am not suggesting that there is a correlation between RULER with RPVOT and TAN as stated by John. My suggestion is to rely on the RPVOT & TAN results and other test parameters for evaluation similar to the ones you have listed.
I can see that you are in Australia, most turbine oils formulated are using a combined additive package.
You should talk to your oil supplier about getting further analysis on the used oil for the antioxidant package on the FTIR. Antioxidant additives such as butylated hydroxy toluene are often used (the phenol you are measuring in the RULER method is most likely part of the BHT as it does contain the phenol group). The FTIR can identify and measure the antioxidant level of old versus new oil, a calculation can be done to determine the amount of top up you need to bring the antioxiant level back up.
However, the success of sweetening is dependent on if you have been topping up with the older style formula or the newer style formula over the years. This may be a reason why you have seen a change in the zinc level as the formula may have changed over the years.
To find out if this is the case, you should talk to your oil supplier.