Just be careful, TallPaul. I always get nervous when cross referencing among three or more filter brands. In other words, if the Onan doesn't directly cross reference to the FL400, but you got there by going first to Puralator, then Baldwin, then ABC, then Fl400 - watch out. Baldwin always gave me fits when trying to cross reference John Deere filters. It's like Wix and Baldwin were recommending different filters that supposedly matched the John Deere.
Again, maybe the thinner gasket might not seal correctly.