Gasoline is sold by government-franchised Pemex stations. The stations are plentiful, in some areas it seems like there's one on every corner.  

A service attendant will pump your fuel, and it is measured in liters (there are approximately four liters to a gallon).  For unleaded gas, ask for "magna sin" (pronounced "seen") and tell the attendant how much you would like to buy, in pesos or dollars.

Oftentimes the attendant will wash your windshield when the car is fueling, if you'd like, feel free to give him a tip (5 pesos, or 50 cents is acceptable, but he'll be grateful for whatever you give him).

Pemex stations often have restroom facilities and sell snacks, as well as having an area where compressed air and water are available for free.

Socoada also has a gas station at its facility on Avenida Colegio Militar, just north of the railroad tracks.  It often is the cheapest gas in town, so it is very popular and often very crowded.  Also, it only has one-way traffic, coming from the
direction of the Avenida Colegio Militar entrance.  

Gas stations typically have an area with free compressed air and water, and clean restrooms.

Pemex Stations

Campo Agricola El Socorro
Alvaro Obregon 71
(637) 372-3276

Centro de Servicios Elva
Alvaro Obregon
(637) 372-0811

Gasolinera El Coyote
Calle 14 #1
(637) 372-3063

Paso del Desierto de Altar
Martires de 1906 #214
(637) 372-2497

Servicio El Dorado
Quiroz y Mora 222
(637) 372-0104

Servicio Garal
Calle 10 #280
(637) 372-7979

Servicio Rivera Hermanos
Avenida Obregon 201
(637) 372-0932

Empresas Bam Lue
Carretera Al Desemboque 1
(637) 371-2049