Dirt is the deadly enemy of a diesel engine, and it comes in many forms. While nobody would deliberately put a handful of sand in an engine, the truth is that you could be doing something very similar every time you add or change lubricants, fill the fuel tank or just start the engine. That’s why there is an oil, fuel and air filter on every engine, and why using the right filter for each is critically important. And the job of a filter is more complicated than you might think.