Pipeline product separators are called "PIGS" They are inserted and removed as needed. Control has to be maintained on both ends for product integrity. A few gallons of "effluent" must be disposed of.
I have seen mighty few trucks that say PEMEX on them. Most are private contractors as are the barges and barge tugs and ship tankers. It is cheaper for Pemex to take vehicle tankers to close to the border destinations. But El Sauzal is the tank farm even though the offloading point is offshore of Rosarito (south). The generation plant and desalinator plant eat a lot of number 6 mexican refined fuel oil. This point also receives Japanese ULSD for pumping to El Sauzal, Tijuana and Mexicali.
The sindicado with the complaints is dedicated "pipa chofers". They are extremely militant as are the refinery workers. There must be issues with Mexican trucks if USA origin delivery is a problem. Theft and contamination is always a concern. If a tanker shows up dirty they will not load it. And like it or not at Otay where the tankers cross is much stricter with Mexican trucks. I learned this while working for Duran Trucking in Tecate. A bonded USA truck can be cleared in 10 minutes, a Mexican truck can take 3 hours.
El Pazo is the main pipeline entry for the entire country. There is chatter about expanding the El Sauzal tank farm in a huge manner, and using it as a transfer point for USA fuel heading south to points in Baja California Sur and the mainland. Remember, USA refined Pemex premium gets as far south as Tapachula Chiapas, by ship tanker.
If AMLO does what I think he's going to do, he is going to jack up fuel prices then essentially build new refineries as annexes to existing refineries. MORENA plans are quite well hidden.
But Mexican refineries are tired death traps. With abundant shale oil and a ready market, I wouldn't not be surprised to find 80% of Mexican motor fuel as being USA refined, in five years. But pipeline thieves must be discouraged.