The Racor 500 Series is a popular drop-in fuel filter.
Fuel filters are usually characterized by two ratings. The first is the micron size. One micron is equal to forty millionths of an inch.
Typical filter ratings are 2, 5, 10 and 20 microns. The micron rating is also characterized by a percentage. In other words, a 2 micron filter will remove up to 98 percent of particles that are 2 microns and larger.
The second rating is the flow capacity, expressed in Gallons Per Hour (GPH). This is the number of gallons that will pass through a filter at a specified pressure drop using clean fuel in one hour.