I had a 2um filter but then changed. I attended a Mack Boring Class last year (spring 08) and the instructor stated Yanmar recommends a 15 um for the primary. The reason is the secondary is 2um and anything less the engine will not get enough fuel on demand. I have a 3jbh 35 hp. I realize it is not a 50hp but you may want to check with a dealer or manufacturer to insure correct filter. I change both filters annually and maybe go through 20-30 gallons of fuel a season.