I agree that there is likely a fine line of resistance that you don't want to cross. I would also assume there is a certain amount of lubrication that will have bad effects. However, I have apparently gotten a pretty good system down, for myself at least. I use a really light "sponge/foam" material that I scavenged from some packing material and only a apply a couple drops of oil on the side of the sponge farthest from the printer itself. I only apply this oil every 50-ish printing hours or so. It's mostly by feel, to be honest, but it has worked wonders for me. I'm sorry you're having trouble. My best suggestion would just be to continue experimenting, but to each his own.
Aside, I've basically decided that the lubrication isn't so much about the extruder gear gripping the filament as it is the friction of the filament inside the feed tube. I honestly have no idea, I just know it works, but the more I think about it, the more it seems to be pretty obvious that the feeding problem came from the friction inside that curled of bowden tube. Perhaps that doesn't explain the success of people that used the filter without oil, but then again, maybe the filament was just dusty and the dust was the cause of the friction. Either way, the filter has probably been the most useful thing I've printed so far and I am thrilled to be able to print things that make the printer itself work even better. Brilliant technology!