It's definitely stainless. I don't think Oscar doubted that, just that it was 304. Whether the oil is impregnated or just on the surface which wasn't cleaned, it could still cause the same problem. Sounds like you cleaned it well before you started welding? With adding the comment about the part be...

First thing I would have done is to check if it was really 304 stainless, which I'm betting it's not. Does a magnet stick to it like it does to steel, or does it feel only the tiniest bit magnetic? Most OEM fuel rails are not SS, I would be willing to bet. Perhaps nickel plated steel or something a...

I know that's true of chlorinated brake cleaner. The lines are always blown out and cleaned and I can't see it as being too easy to keep a chlorinated gas around while welding but you never know. You damn sure don't want to be wrong, I guess.

I'm hoping one of you seasoned pros can offer some clues for the less experienced people, like me. I'm not completely sure what details matter vs what don't so forgive me if me trying to be too detailed becomes annoying. I use an Everlast 350amp TIG machine and custom water cooled torch rig. 3/32 2%...

Are you sure that you got the gas lens and cup for a WP26?
The gap definitely matters. It'll draft air in from the back side and contaminate your welds if it's not perfectly sealed up.