I have both P21 and MCH6 stainless filters. Both off ebay and appear to work fine. I double filter anyway thus no issues via my fills. I find it amusing that people scare monger to try and maintain market share. If you use reasonable MS and AC, its not an issue. People try and scare monger that its a dark secret and you need to pay $65 for a cartridge or you will die. The reality is that if you know the science you are fine. MS will remove 20% by volume of water and AC 7.5% oil absorbency. That's scientific fact. The rest is scare mongering. Designed by people with vested interest. I measure my filters on a regular basis. Thus ensuring that they don't break through. People who use the 1 filter a month logic are fraught with danger, what number of fills did they use during this month, 1 or 10,000. That sucks. On a weight basis soon as you reach max weight the filter is dead.