Originally Posted by Bill_W
Seeing RP filters and M1 filters inside guts, looks like they are the same except for the pleated filter media and the purple check valve color...
Nope.
M1 uses a synthetic blend media, RP uses a wire backed synthetic media. Wix XP (and Fram Ultra) also use a wire backed synthetic media.
IMO, a M1 filter is overrated and overpriced for what it is. Some knock the Ultra for the thinner can than others, but no reported issues with it.
The Wix XP does not have published ratings.
As other have said, the Ultra is the best bang for the buck. Lowest cost, best filtration, and readily available.