What type of oil are you using on the prefilter? Regular engine oil? I've used good ol' 10w30 before and did find it was easy to overoil the filter. Now I use Air Filter oil, the type that they use for motocross bikes air filters. A mechanic at a local shop told me that it's like a grease mixed with an alchohol carrier so it flows easily as you pour it and work it into the filter, then the alcohol evaporates and leaves a very sticky heavy oil on the filter that really grabs the dirt. The only downside is that the stuff is so sticky, about the only way I've found to really clean a filter off once you've used this stuff is with gasoline, so you have to keep an eye on the filter material to make sure the gas doesn't start to damage it. I use the the Yamaha (yamalube) brand if it makes a difference.