The world can be a dusty place and 'Africa Twin' air-filters are not available at every street corner. Taking a dozen with me would have forced me to use a sidecar or trailer, but fortunately  K&N filters makes a washable air-filter that will last up to one million miles (1.000.000!). If I had to choose again I would probably get a foam filter. It's less vulnerable and can be oiled with any engine oil. The K&N filter needs special oil and that can be a problem.