Recently there have been posts regarding dirty fuel and water in tanks and gas cans. (Just let leave a gas can outside at -5 and notice the ice in the bottom).
Many years ago I purchased a fuel filter funnel and have never had an issue with dirt or water getting into fuel tanks since. I don't know the scientific reason it works but it does. Removes dirt and water from all types of fuel and doesn't slow the flow compared to a non filtered funnel.
"West Marine" supply sells them and I have seen similar funnels sold elsewhere for about the same price.