Nowadays they probably are only found in certain utility vehicles, earlier, the oil-bath air filters were also widely used in motor cars. The above example is from the VW-Beetle, and had to be cleaned every 2500 km and be supplied with fresh oil. The filter was laid onto a wire mesh, and thus, drew off the dust from the inflowing air.
An interesting point in this example is, next to the the normal intake duct on the left, is a second duct underneath it, for the pre-heated air. In this case the flap was thermo-statically controlled to take in cold air when the engine was warm, and warm air when the engine was cold. Since on cold inlet manifold- and cylinder walls, as little petrol as possible should be allowed to settle. With a warm engine however, a good filling, through the cold air, was required. Other examples were not as flexible. Either the driver had to position the flap for summer or winter operation, or, through a weight on the flap, only a certain, basic amount of warm air was taken in. 10/09