Heyas!
Yep, you are right that these tags are unique and not great.

For adding fuel tags, I used a combination of the ones that are in the JOSM built-in preset and some taginfo search as guidance. For "fuel:blue+", "fuel:diesel_e+" and "fuel:diesel_e+10" I assumed that these are Repsol-exclusive fuel types whose name/type is this, or industry-wide ways of referring about them, so I added them as such.

I'm doing edits on other gas stations in Valls, so I'll revise the fuel tags of this station and the others, and I'll try to fit them more common values.
If I don't find a taginfo equivalence, I'll note it in a fixme= or make a nearby note.

Thanks for the heads-up!