Dear Kaye,
The primary cause for increasing acid number (measured as mgKOH/g) is hydrolysis. Hydrolysis occurs when even small amounts of water are present in the Fyrquel fluid. When making Phosphate Ester fluids you can look at the following chemical reaction:
Acid + Alcohol -----> Phosphate Ester + Water
The problem here is that this reaction can reverse and hence the reason for increasing acid levels. The reaction is accelerated by heat and acid. You can double check with your fluid supplier but I think it would be good to keep the water content below 500 ppm. The only way to do this is by using vacuum dehydration. Water absorbing filters or coalescers will not allow you to reach this goal. As far as recommended AN, normally, it is good to keep this fluid below 0.2 mgKOH/g. New oil is typically 0.02 mgKOH/g.
Fuller's earth is an acid scavenger, but contains extractable metals that can react with the acid. Fuller's earth contains Ca and Mg so you need to be careful about these metals. Again, you can check with the fluid supplier but I don't think they recommend using Fuller's Earth when the AN is above 0.25 mgKOH/g, primarily because of the release of these metals into the oil.
There are other means of reducing the acid without harming the oil or releasing any metals in the system. You can use Ion Exchange resin that will reduce your acid level and have no detrimental affects on the oil.
I hope this was helpful