The filter I used was from McMaster Carr...part number 4369K31. I tested the amp draw with and without the filter and it was 0.1 to 0.2 amps higher with the filter in place. That seemed reasonable to me so I accepted it as "Good Enough". It's been in service for about 4 years now on my Gast 0523.
Here's the link...
https://www.mcmaster.com/catalog/127/428I chose this filter because it was the same pipe size that the pump offers at the connection. McMaster has higher CFM rated filters, but you have to bell up to 1/2" pipe size and that seemed unreasonable.
BTW, the filter comes with an element included so don't buy two replacement elements thinking you'll use one and set one back as a spare.
