Chevron maintains one “Standard” brand station per state so it doesn’t lose the trademark.

This particular area has standards.

Standard Oil = S.O. = “Esso” in some parts of the world (at least when I worked at an ad agency years ago).

Standard Oil was in the Midwest but all under a parent company. I believe Standard. Chevron and Amco (a few others?) had different names to avoid becoming a monopoly. IIRC. Maybe I'm old and just making things up.

As others point out, it’s a trademark thing. “Standard Oil of California” is the OG, renamed Chevron Corporation in 1984.

Standard Oil = S.O. = “Esso” in some parts of the world (at least when I worked at an ad agency years ago).