IMO for visibles it was almost always water decals.
Answers assume you want to do the pumps period correct - when they were released...
In this case, 1923-28, the Texaco could go with a curved round Texaco porcelain sign with a white rectangular decal saying "Gasoline" below it, or a similar Texaco decal. Pump would be red and globe a cast or etched Texaco star. From 28-32 they changed to the curved round "Clean Clear Golden Texaco...porcelain sign and also available was metal band globes on the west coast.
Unfortunately not much is available for Mobil of that era... So you may end up ordering decals from someone who makes them...
"Mobilgas special" came around in 1936 - but of course you can do it anyway - but Vacuum had a Mobilgas Ethyl 1926-32. White pump with a 10 inch Black band around the base. Rectangular Mobilgas decal with an ethyl logo on pump.
Also t-way has a shield ethyl decal.
http://www.t-waysgraphics.com/decalsMN.htmSource: PCM guide to gas pump restoration 2nd edition.
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Claes