ABSOLUTE ENGINE PROTECTION | MINIMUM FLOW RESTRICTION | THE BEST DIESEL-GRADE FILTER AVAILABLE | GUARANTEED PERFORMANCE | Efficiency without Restriction Anyone can design a filter with high efficiency. But only aFe's NF21 full-synthetic media achieves extreme efficiency without flow restriction. Because clean oil means nothing if it can't reach vital engine parts. 5,000 Miles of Ultimate Protection Most particles larger than 40 microns are trapped by air filters or cannot pass by an engine's piston rings and enter the oil system. Particles smaller than 5 microns pass through an engine without causing damage. It's the particles between 5 and 40 microns that penetrate a vehicles defense, contaminate engine oil, and ultimately cause destruction. aFe Pro Guard D2 oil filters with NF21 full-synthetic media trap these particles without sacrificing critical oil flow for the ultimate engine protection.