A picture tells a thousand words.
Above Left: the first air filters, were replaced at 5,000kms
Above Right: the same filter at 17,000kms
Below: the same again at 45,000kms
When the filters were replaced at 5,000kms, (the Africa Twin has two) another pair of Genuine filters were used along with a pair of UniFilter pre-filters.
Between the first change and the most recent inspection, they'd been checked a few times and the pre-filters only required cleaning twice in that time, which included a 30,000km trip around Australia.
A pair of genuine filters, and aftermarket for that matter, aren't cheap (around $200 a pair) so anything that can extend the life and reduce costs is a winner.
There's a few aftermarket filters available, UniFilter do a full kit, K&N, Twin Air, HiFloFiltro and a few others, in a range of serviceable foam or direct replacement dry filters.
Although each time the Genuine filters were checked, the airboxes were spotless, so despite the advantages of serviceable ones, it just seemed obvious to keep OE filters in there.
Mud, dirt, outback roads and bulldust, the airbox has remained spotless using the pre-filters and OE main air filters.
And at only $22.50 it's a lot better than replacing main filters each time.
Fitting the pre-filter is simple, it does require the snorkel to be trimmed though. Once done, the pre-filter is simply rolled over the end of the snorkel and zip tied.
The pre-filters didn't survive the last wash though, but a hot glue gun will sort that easily.
No matter how you look at it, these things save your wallet and helps keep your bike breathing clean air.