[billwot]

Originally Posted by Tcoat

How is this even a question? read the manual. It will tell you what you should and must use.
The octane levels are for reducing knock ONLY! Higher octane fuel does not give you more power really. What it does is allows the engines with learning ECUs to adjust the timing higher. This is where the "more power" is created.
As a society that identifies higher numbers as being stronger or better I think many people totally misunderstand octane rating. High octane fuel is intended to reduce preignition in high compression engines it is not stronger or better than low octane fuel in any other way. To grossly over simplify what is happening the higher the octane fuel the harder it is to ignite not the other way around. Sure you can run high octane in a regular engine but it is not doing a single thing for you. You should not run lower octane in a high compression engine though since you run the risk of damage. New engines with self adjusting timing this risk is not huge but run too low in an older engine and you better be good at fixing it.