What apparently few people understand about multi-stage filtration is the process called "caking". Edge type filters (Racor for example) accumulate fine particles on the surface which enhance the filtration. If you use a 3 micron element on the primary filter virtually all the particles get trapped at this point and the secondary filter never gets any caking. By using a 30 micron filter as the primary, the smaller particles pass thru to the secondary thus making it more efficient. The caking process is most evident on swimming pool filters that use diatomaceous earth. The filter is basically just a screen and the diatomaceous earth is spread across the surface of the filter and is sucked into the filter screen where it cakes on the surface and becomes the filter. Continue using either a 30 or 10 micron primary filter. The article in PassageMaker Magazine was incorrect and most engine manufactures specify the higher rated filters for primary application. Mike, I fully agree with you, the passagemaker article was written by the editor, Bill P. He and I have totally different cruising philosophies, and look at boating very differently. His professed boating wisdom, or lack of it at times, is passed down as if spoken by god himself. I don't mind speaking ones piece, I sure do enough of it. It's when you set yourself up as an expert and then gather a flock who think that what you say is the only truth. I have a problem with that. Since Passagemaker was just purchased, perhaps Bills days are numbered. Jim Clausen