This sounds like confusion over the many ways of rating octane.  In the old days, there were two ways of rating octane, research octane and motor octane.  Most car companies (VW included) specified octane by the research method.  When the US government decided to require labeling of octane number on the pumps, they couldn't decide which method to use, so they created another one! (its actually the average of the two traditional methods)

So, the Pemex gas is probably supposed to be 91 research octane = 87 US government octane.

As a side note, the sticker on the Vanagon that says to use 91 octane (RON) fuel, really means to use US 87 octane regular gas.

Bill

>Kurt,
>I am also headed to Baja in April.
>I have been reading, and some claim that the actual = octane is less than
>stated by Pemex. In fact, some recommend taking = along octane boosters to add
>to the gasoline.

>>I'm bring my syncro to Baja for the month of = April and have heard that the
>>only unleaded gas available is 91 octane.  = If this is really the only
            octane
>>available, it seems like I should tank up on 91 = in  the states and get the
>>timing & tune redone (I use 87/89 = now).  Am I correct?  It seems like I
>>would need to adjust the detonation for the van = to run smoothly on 91.