San Juan is a city in Argentina and is the capital and namesake of San Juan province. The Ruta Nacional 40 passes through it.
South towards Mendoza
Take the Ruta Nacional (RN) 40 southward. There's an Oil brand gas station about 15 km south of the city center, on RN 40 at Calle 9. For several years the RN 40 was closed between Calle 5 and Calle 8 in its southern approach to San Juan, but as of February 2019 it should be reopened, as a brand-new stretch of limited-access highway.
There are two ways to get to RN 40 and Calle 9 by public transit. Both leave from the central bus station, Terminal San Juan, located at the intersection of Calle Estados Unidos and Calle Sante Fe Este. Most local buses pass by the terminal at some point in their run, and if not any driver of any bus will be able to inform you of whatever transfer is necessary. Do not expect much help at the station, however, as there is very limited information available on local bus routes or departures.
Certain runs of bus number 24 will go right past the Oil gas station along RN 40. However, be sure to ask, as others will take Mendoza Sur, 2km to the west. The 24 bus does not run very frequently, especially on weekends, and so if may be worth it depending on the schedule to take one going down Mendoza Sur, get off at Calle 9, and walk to RN 40. If you are catching the bus from Terminal San Juan there will invariably be people waiting who will be able to tell you which route the next departure should be taking.
Regardless of route, if the next departure of the 24 bus is a ways off you can take the 16 bus, which departs more frequently and always runs down Mendoza Sur. That means walking down Calle 9 to RN 40, but it's only 2km. The 16 bus departs from the bus stop in front of Terminal San Juan on Calle Estados Unidos.
If you should arrive in good time for a 24 bus departure, check to see if it goes all the way to Media Agua, and consider taking it there. That is a large crossroad with crossing light, shoulder, and an Axion gas station that sees a fair bit of truck traffic. Moverthathoof had a ride from there right to Mendoza just minutes after jumping out of their ride from the Oil gas station.