A practice you may want to consider is to send a new sample to your lab to find what the new lube sample starting acid number is. An increase of 0.2 should raise concern and be monitored, and an increase of 1.0 should be considered critical. You also want to ensure that you are following a good trend with your results. You should also try to find out why your AN results are high. Is water present, increasing the possibility of corrosion? Was the system run hot? There are several possibilities. Being and ISOVG 68 fluid, was the viscosity high on the fluids with higher acid numbers? I'd also expect that the lab ran FTIR and reported an oxidation value. I'd expect this to be higher than the previous sample as well. There are so many factors to take into consideration. I'd definitely begin by sending a new lube sample to your laboratory for a baseline of the acid number.