Correct. The PCM will run with more advanced timing assuming you're using a higher octane.

You guys made me go look something up. I remembered reading in my Aurora GM service manual that it had a list of what I could add to the fuel and what max % per tank.

Alright, I could of swore tuleane was on there put it'* not. Now from GM

-Methl Tertiary-Butle Ether (MTBE)
Fuel containing Methyl Tertiary-Butle Ether (MBTE) or grain alcohol man be used, providing that there is no more than 15% alcohol by volume.
-Ethanol
Fuel containing ethanol (ethyl) or grain alcohol man be used, providing that there is no more than 10% alcohol by volume.
-Methanol
Fuel containing methanol (methyl) or grain alcohol man be used, providing that there is no more than 5% alcohol by volume.
-Notice: Do not use fuel that contains more than 5% methanol. Use of a fuel (gasohol) that contains more tha 5% of methanol can corrode metal fuel system components and damage plastic and rubber parts.
-Notice: Do not spill fuel containing alcohol on the vehicle. Alcohol can cause damage to the paint finish and the trim.

So, I've thought about adding something to my gas, lately. I sometimes get 100 octane at two different places. Question now which is better, one says they add MTBE and the other place is methanol. Roadpilot and a citgo. both $3.10ish gallon.

So I'm glad I just looked that up in the olds aurora manual. I just realized that, Actually forgot all bout it that the maunal is an Aurora/rivera. I have L67 info too. Lots of reading to do. The info I posted above was for the 4.0, the info wasn't there on the L67.

my 4 cents.