I think you have to realize that the manufacturer is making a general recommendation for a engine system that does not use a vacuum gauge and that the owner expects to be told in advance when to change the filters without any knowledge of the quality of the fuel they are using. Given this scenario I would recommend 30 micron filters for the primary. I think people have to apply common sense not just blindly follow the recommendation. I am sure that if you went back to the engineers who wrote the specs for the engine and told them that you were using a vacuum gauge and watching it, then the recommendation would change. This situation is similar to oil change recommendations. They are based on time and hours unless oil analysis is being utilized. In other words monitoring a system trumps hourly recommendations. I use 2 micron filters as primary because I believe they keep my fuel cleaner than 10 or 30 micron filters would, especially because my engines recirculate a lot of fuel. They last for several seasons and I keep an eye on them. If the vacuum exceeds 6in, then an alarm goes off on the bridge. I have a neighbor who has to change his 30 micron filters several times a season. He is not a good candidate for the 2 micron filters. Given the fact that these filters have the same flow rates, each owner has to evaluate the situation and make their own decision. Personally, I want Racor to come out with a 1 micron filter. Frank Burrows Destiny 1979 43' Viking MY Piney Narrows Chesapeake Bay