amy w,
Both options could be effective. Bare in mind that the amount of varnish particles in oil depends on oil’s solvency characteristic; higher solvency – less varnish particles and vice versa. Also, temperature is a factor; warmer oil – higher solvency. Small presence of water in oil does not affect performance of a low-flow cellulose fiber filters; actually they would absorb it and keep the oil dry. On the other hand, water in oil (better say moisture) decreases the efficiency of electrostatic filters, unless they include a cellulose fiber in its configuration (I have seen such combination). Considering this situations, in my opinion, a low-flow 3-micron cellulose fiber filters are better suited (and are cheaper) in conditions where the oil temp is bellow 75 C. This is due to coagulation of varnish particles phenomena occurring at lower temp. Electrostatics filters are the answer at higher temp, where they are more effective of capturing individual varnish particles.