You will find Castrol dino motorcycle oil 20w-50 even in some grocery stores.
Look for a Japanese bike dealership like Yamaha or Honda and you might find synthetic but it will hurt the wallet.
The easiest route is to a MotoMundo location as they normally stock synthetic oils. They don't sell a lot, but they stock it.
There are 3 MotoMundos in Merida, Yucatan.
It is a fairly safe bet that among the 3 they will have enough of a synthetic brand and blend to see you through.
Let me know if you are coming back through Veracruz.