AMSOIL Ea Motorcycle Oil FiltersA reader asks: I’ve been researching oil filters for my Harley-Davidson motorcycle. Harley-Davidson has oil filters that will stop particles down to 5 microns, while AMSOIL motorcycle filters only stop particles down to 20 microns. I thought that AMSOIL oil filters were supposed to be one of the better filters out there? Why are Harley filters so much better?

Our answer: The oil filter micron ratings provided by both AMSOIL and Harley-Davidson “Super Premium5” oil filters is like comparing apples to oranges. Each company is measuring their micron ratings on completely different scales. Therefore, the micron numbers are not at all comparable.

AMSOIL offers their motorcycle oil filter ratings on an “absolute” scale. Based on this measurement, their motorcycle oil filters are 98.7% efficient at stopping particles that are 20 microns in size or larger. This is a considered to be a very strong efficiency rating.

For reference, a decent automotive oil filter would stop 98.7% of particles 30 microns or larger.

Harley-Davidson provides their micron numbers on an “nominal” scale. Their nominal claim of “5-microns” with their “Super Premium5” oil filter is not telling the whole story. Filters makers claiming ultra-low nominal micron ratings aren’t mentioning the “efficiency”.

Let’s be clear. Harley-Davidson Oil Filters are definitely not removing 100% of particles down to 5-microns. This would make the oil filter so restrictive that it would go into by-pass mode every single time you rolled onto the throttle. Under most all riding conditions, that would be like having no oil filter.

What is the Difference Between “Nominal” And “Absolute” Oil Filter Ratings?

Absolute filter ratings are determined under controlled testing. The absolute filter rating is shown as 98.7% of particles of a stated diameter being stopped by the oil filter. In the case of AMSOIL Motorcycle Oil Filters, they stop 98.7% of particles measuring 20 microns in size or larger.

Nominal ratings such as the 5-micron rating touted by Harley-Davidson has zero meaning without mentioning efficiency. Chances are high that the H-D filters are about 50% efficient at 5 microns. Where a Harley 5-micron oil filter would score in an absolute filter micron test is anyone’s guess.

What Is a Micron?

A micron is a unit of measurement that is 0.001 millimeters or 0.000039 inches. For example, bacteria are 2 to 3 microns in diameter and a red blood cell is 5 microns. The diameter of human hair averages 70 to 80 microns.

Bottom Line

To sum up, Harley oil filters might be well-built and near the top of the heap. But it’s very hard to tell. The nominal 5-micron rating touted by Harley-Davidson has no meaning without mentioning efficiency.