One counterintuitive aspect of SCR systems is how sensitive they are to tiny contamination in the urea solution. Trace metals or even a small amount of tap-water minerals can crystallise at the injector and form deposits on the catalyst, increasing backpressure and dosing errors. In Nordic use with road salt and slush around the filler neck, cleanliness during filling matters as much as concentration. Use only sealed containers, dedicate funnels, and rinse spills with demineralised water; mixing with fuels or coolant is harmful. These basics apply when topping up with AdBlue.