You can go to the Wix filter site and look up the filter for your application. To find the Napa Gold equivalent, drop the 5 from the beginning of the Wix number, keep the last four digits and you have the NG number.
The XP is Wix's synthetic media filter offering. In all likelihood same as Napa Platinum. Not the same as NG, which is advertised as a cellulose blended with some synthetic fibers.
The link shows what Wix says about each filter offering. I'd say unless you're doing 10k+ mi ocis regularly stick with the NG.
Napa Gold is Wix. Just drop the first digit of the Wix part number to get the Napa Gold part number. For example:
Wix 51348 = Napa 1348
Wix 46017 = Napa 6017
Wix XP is Napa Platinum.
To get the Platinum part number, add a 4 to the beginning of the Napa part number. For example:
Wix 51315XP = Napa 41315
Wix 57186XP = Napa 47186