Joy,BVA(Dave?) - I would worry about two things with the problem you have described. The plugging of the filters is a serious issue that suggests major problems in the compressor or system.
Excessive heat at a metal wear surface can decompose the POE and can cause the viscosity to drop. Thermal decomposition can result in cracking off small molecular weight fragments from the acid chains, and can best be seen by analyzing the refrigerant looking for the presence of small MW hydrocarbons. You may also find high amounts of wear metals in the POE from the metal - metal contact. The TAN number does not always go up a lot due to thermal breakdown. The filter plugging may be wear debris.
If the POE decomposition is due to moisture (hydrolysis), then carboxylic acid salts may be the source of the filter plugging. The viscosity will generally drop as a result of hydroysis as well. The salts are generally wax-like semi-solids. The TAN number will generally rise quite a bit to the 0.2 limit or higher. You may also see corrosion metals in the POE sample as the water and acids go to work on the system.
The absence of any decomposition products coupled with a viscosity drop is either from improper refrigerant stripping prior to testing the viscosity, or the addition of a lower viscosity POE to the compressor. Infrared anaysis of the POE is a good tool to see if the POE is mixed. You can also see large refrigerant bands in the infrared spectrum if the sample contains more than about 1% residual refrigerant. Since you have filter plugging, I would not suspect POE mixing or dilution to be the problem.
Bottom line: if the filter is plugging, then change out the POE lube. The breakdown of POEs are autocatalytic, which means the presence of POE breakdown products results in faster decomposition of the remaining POE. Add filter dryers to help remove any acidity to keep the system clean as long as possible.
