I live on a gravel road 1/8 mile long, where I sometimes have to drive in gritty dust, coming and going ... it gradually rises up to coat my engine compartment with shiny grey dirt, rock dust. And when the engine is not yet warm, it's temperature sensor and valve directs air intake to be pulled up from the exhaust manifold area, down lower on the car's engine, closer to the dust coming up off the tires and road. Also there's a lot of pollen here in the Spring & Summer which helps clog it up. So I use upgraded air filter changes, every 10-15,000 miles, BEFORE I can actually see a lot of dirt on the paper. If I see some dirty areas begin to appear in the bottoms of the folds, for me it's time to change. I'm sure it's mostly nano-grit trying to get through. Maybe I'm obsessive but my 16-yr-old engine is still very tight, perhaps as a result of this and other upgraded maintenance intervals.Some people claim that a filter can filter better when it's becoming dirty ... dirt...