Most larger towns have Pemex stations with 89/91 octane unleaded, while the low grade is 87 octane. (Ironically, low grade in New Mexico, USA is 85 octane...).
I've never had a problem with bad gas, and have even bought it out of 1 gallon bleach jugs before. If you are worried, take along a couple of filters for the bike and a funnel with a fine screen.
Or, of course, you could just take an old, tried and true Honda XR650L, which I think would run on anything sometimes...
With a little love and luck, we will get by.
2003 BMW R1150GS Adventure
1983 FJ45 Land Cruiser Troop Carrier