Volks,
Pemex is the gas, the only gas, available in Mexico.
Their octane rating is figured differently than our's here.
For a Vanagon that normally does fine on Regular gas here,
I find my Vanagon needs the intermediate gas in Mexico.
Most stations in Mexico offer as many as three different
ones.
Never use Regular unless the others are available, which
happens frequently in some areas where they either are not
yet set up for higher octane gas or are OUT of what you need.
When you are forced to use Regular Pemex, it is good to add
an octane booster to a fillup.  Normally these octane boosters
are good & available in Pemex stations, but not always in stock & they cost
twice what they cost at Wallmart.  So it's good to
carry some.

Pemex stations are notorious for having water in their gas!
No problem, I just carry lots of STP water remover @ about
$1.27 a bottle at Wallmart & add it periodically.  You will know
when your engine needs it--it will become quickly sluggish & lack it's
normal power.  Just dump in a bottle of the stuff & all is well
again.

Don't worry about Mexican gas--it's fine if you can fill up with
the Premium.  Or if theyre out of Premium, but have Regular &
Super, just fill up half & half.  Use the STP octane booster when
you've got no choice but to fill up with Regular.  It works!
Mexican oil is good quality & comes in many weights.  I prefer
to carry my own Castrol 20/50--but would't hesitate to use
Pemex oils if need be.  In Mexico, it is normal to be given a free
oil/filter change for the cost of their gas & oil.  And using my own filters
& oil, I always found someone on hand who would
*happily change my oil/filter for the cost of a pack of cigarettes.