Wix and Napa Gold are the same thing, but not true of Napa's lower level offerings. Same can be said for Car Quest; their top shelf filters are rebadged Wix, IIRC. Both retailers have lower lever offerings that might be made by Wix (or others) but not to the same standard as Wix brand name stuff. But their top shelf stuff is truly rebadged Wix.
So, in Napa Gold, a 1515 Napa Gold is the same as a 51515 Wix, and so on. Just drop the first number from Wix and you get the Napa Gold filter number.
There are some rare oddities that I can't explain, but have noticed. For example, typically the Wix/NG stuff has silicone ADBVs. But there are a few filters that break this tradition.
Take for example the contrast of 1358 and 1365. Same gasket size, same thread pitch, etc. The 1365 is just a tad shorter, and has a silicon ADBV, while the 1358 has a nitrile ADBV, and is a tiny bit taller. I choose the 1365 over the 1358, even though I lose a small amount of media; makes no difference in my GL1800 5-6k mile OCIs. 1365 is actually less expensive as well!