Plants are open Mon/Fri 7:00AM to around 7 (mas o menos).  Closed Sundays, and all Mexican holidays. Carry a calendar.  Gas is usually sold in Kilograms even though they may have a liter dispensing meter.
a) 2 kilos per U.S. gallon,  b) 2 kilos = (4) 3.78 U.S. quarts

Here's how I barter at Guerrero Negro and Santa Rosalia: Clean off and make legible the data on the
liquid fuel storage tank so that they can read the capacity of the tank(s). Clean off, or replace the liquid
level gauge so that they can see the needle inside the clear plastic housing. You, yourself, need to be self
educated in estimating the quantity of fuel remaining in your tank so that you can make a guesstimate as
to the correctness of the plant operator's "guesstimate" ALWAYS, ALWAYS, ALWAYS, ask for a
presupuesto, (estimate) first before they start pumping.

Mexican fast fill nozzles are about 6" shorter than most U.S./Canadian fillers. Make sure that your access around the fast fill valve is clear, or if it isn't make an extension. I always carry a POL to Fast Fill adapter.

Bulk trucks (the dispensing kind with six wheels) work like our trucks. They have meters and pumps.
They are allowed to fill your vehicle and commonly charge a price that is exactly in between domestic and motor fuel price per liter. They ar selling motor fuel on the sly, so the extra amount over and above the top of domestic fuel price is their "tip", or service fee.
I ALWAYS pay in pesos. Dollars will result in a 20% price penalty. They do not accept Traveller's Cheques.
Be prepared for spot shortages Dec/Mar. Tanker trucks (18 wheelers) bring in gas from Mexicali, or over from Topolobampo to La Paz, then north, on the ferry. Shortages last 2 - 4 days. Best days to buy gas, Monday afternoon - Thursday
It is up to you to keep from blowing up when refueling. Douse the refrigerator and hot water heater, before you start fueling.
It is common for someone to fuel you up, then write out a receipt. You'll present one copy to the cashier
at the window, take another back to the fuel person as proof of payment, and stuff the third copy in your
pocket as a souvenier.
 

 There are two grades of unleaded gasoline. Regular unleaded is found in the GREEN dispenser pump and is called MAGNA SIN. Super unleaded goes by the name PREMIUM and is dispensed from RED pumps. Look below the station's large banner "Pemex" sign and you'll see smaller color-coded product signs. Black is for diesel, green for Magna Sin, and Red for Premium.

 México manufactures about sixty five percent of its gasoline. The remainder is purchased on the spot market in the USA. Premium is manufactured in the USA as well. Thus, you will encounter subtle but detectable variations in the quality (octane level) of Magna Sin gasoline. Diesel fuel is refined in México.

 Magna Sin replaced "Extra" unleaded in the early 1990's and is definitely an improvement over the early unleaded fuel. To discourage tampering, PEMEX added green dye to Magna Sin, which is still being done today. As noted earlier the quality of Magna Sin varies according to its source. Méxican refined gasoline is poorer in quality than imported fuel. US fuel exhibits 87 octane while its Méxican counterpart is two or three points lower. Magna Sin does not contain MTBE or alcohol.

 Someone has to haul the fuel to the station from the tank farm in La Paz or the tank farm in Ensenáda. There is a chronic shortage of tank trucks. Second, high winds may delay the Topolobampo / La Paz barge (the ferry boat as well). Remote areas suffer a lack of stations because few people wish to relocate there to run them. Señor Sanchez (SAHISA) closed the station at Punta Prieta several years ago (reportedly because of employee theft and embezzlement). Pemex reports a six-fold increase in total fuel consumption over the years 1980 - 2000. Fuel consumption doubled 1994 - 2000. As more and more vehicles transit the peninsula they are going to tax the fuel supply to the limit and shortages are likely for the foreseeable future.

 The best way to deal with dirty fuel is to add an after-market fuel filter in the line between the fuel tank and engine. Don't bother with an inline filter as they will plug up with just a minor amount of contamination present. The best filters are for diesel fuel (can be used with gasoline), and are larger than your engine's oil filter. The filters withstand high pressure from fuel injection pumps in the gas tank. The best part is that the filter capacity is huge and the filter media is extremely fine. Look in your local Yellow Pages under "Filters". Ask about easy to mount filter mounting housings. My preference in filters leans toward the selection of a GMC "Secondary" fuel filter.

 Years ago I came up with a set of rules and conduct that has just about eliminated the possibility of getting cheated at the gas pump. Here’s how I do it:

I exchange enough Dollars for pesos at the border to pay for gasoline all the way to my destination. I make every attempt to get the pesos in small denomination bills so that whenever possible I can pay with exact change.

I installed locking gas caps on all of my tanks. The keys fit onto the same ring as my car keys. No one will steal a locking cap minus the key however sales of heisted (expensive) late-model non-locking gasoline caps are a minor industry.

When I pull into a station, the attendant has to wait until I unlock and remove the gas caps. This allows me to verify that the gas pump meter is starting with all zeros.

I will observe the meter on the gas pump while my tanks fill. My vigilance is not disturbed by a horde of windshield washing rag-a-muffins, beggars, or the tamale lady. I let my partner deal with distractions.

When the filling is finished, I note the money amount displayed on the pump and say the quantity aloud. Even when said in English the fact that I have so noted the amount will be apparent to the attendant who will assume that I am keenly aware of the amount owed.

I'll make every attempt to make exact change for the transaction. When that isn't possible I will announce the amount of money that I am handing over. Again even if this is spoken in English the attendant will know that you are staying on top of the situation. 

If I have tendered a large bill and have a lot of change coming, it may be offered in the form of high denomination coins as well as banknotes. I will walk near the hood of my vehicle and then receive the change from the attendant. I will use the hood as a counting table to facilitate the accounting of the change.

Most Important. Only some attendants are crooks. Most are honest. I always greet an attendant with Buénas Dias (and mean it). Then I plaster a smile on my face which remains until the transaction is finished. Honest attendants eventually become very tired of being treated like the kin of Ali Baba. Frowns, suspicious glances and curt utterances will eventually sour even a good-natured personality. An attendant whom is subjected to an endless stream of abuse will probably give in and live up to the reputation¾wouldn't you?

 In a curious aside, a presidential candidate recently announced that he would slash the retail price of gasoline by thirty-percent if elected. This first-of-a-kind admission that fuel prices are out of line may be nothing more than rhetoric by a hungry politician. Then again because the statement came from the candidate most likely to be the next president of México, it bears watching (but don't hold your breath).