Is it metal or horsehair? If metal, any good degreaser should work (I've used Awesome on metal range hood filters and it works great).
Here's one guy's regimen for horsehair:
Oil Bath Air Filter Cleaning
by John Anderson
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I read an SAE report a few years ago comparing oil bath wicking filters to some popular pleated paper and oiled gauze element filters. The oil bath was superior on all counts. VW knew what they were doing. It lasts forever and maintainence isn't really all that involved.
I had to wash the filter element on the '70. I sat the top part with the horsehair in a 5 gallon bucket of kerosene for an hour. As it became evident I'd go through kerosene forever, I went to TSP and very hot water. I went through 3 water changes then finished back in the kerosene. It was AMAZING what came off the horse hair, and scary. But when I was finished the hair appeared brand new and clean as a whistle. After the last water wash I hooked a compact hair drier into the outlet with duct tape and let it blow dry for a good half hour. The whole thing was toasty warm with no moisture left. Then I dipped it once into kerosene, dried it, then reassembled with a cleaned bottom half and new oil.
More good poop:
Yesterday's Tractors - Oil Bath Air Filters
Steve