You can let the oiled gauze filters get pretty dirty on the outside. In fact, up to a point, the dirtier they get the better, as the crud makes a barrier against fine dirt particles. How dirty is too dirty? That's a tough question, as even when the filter is so dirty you don't want to get near it, it's still flowing sufficient air. Cleaning time is a guesstimation.
No air filter element likes water. While the oiled gauze ones seem to tolerate and repel water better than paper ones, you should really find some other way to keep water away. This can be as simple as a nylon stocking over the opening or as complicated as a snorkel. For open-element air filters that sit directly on top of the carburetor or throttle body, there are special tight-weave nylon outer filters that help prevent water entry. It could be necessary to make some type of a shield around the front half of the filter to prevent entry from splashed water during a stream crossing. Attach the shield to the air cleaner's lid, or even to the hood itself. On my own Jeep the shield is fastened to a crossmember that goes across the engine from the shock towers.
A good air seal between the filter and its container and the carb/throttle body is important. I like to use a long-fiber grease on both the gasket and the filter's rubber seal from lid to base to ensure no dirty air can find its way into the engine.
Next time you're doing maintenance to your rig take a few minutes to look at the filter. Time spent on it is equal to money saved on engine repair.
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