decreasing TAN as oil ages?

Most likely acidic additive depleted (such as ZDDP or other additive that reacts with base titrant - TAN decreases) and conditions were not severe enough to cause oxidation - no carboxylic acid increase - no increase in TAN

thanks for the help Bob K.

One more question. I also recorded the FTIR spectra of the oil samples and attempted to determine the change in carboxylic acid. The only peak in the range typical for oxidation products was at ~1744cm-1 (was looking in the region of ~1640cm-1 to ~1750cm-1). From evaluation it seemed that the carboxylic acid content increased as oil aged. (Thereby adding to my previous confusion regarding TAN). Is it possible that what I recorded at 1744cm-1 was not carboxylic acid but an ester with the carboxyl group (-COO-)? Would an aged oil tend to show an increase in ester content?

You can get esters, ketones, aldehydes all of which have C=O along with carboxylic acids(acids also have -OH which have peak in the 3500-3200 cm-1 region). But sounds like the change in the 1700 cm-1 region was minimal so the depletion of acidic additives could more than compensate for small increases in carboxylic acids. In the 1640 cm-1 region salts of carboxylic acids and unsaturation also show up both of which can increase with oxidation

very late reply here from me, but just wanted to say thanks Bob K. for all the help