RON.....Reasearch Octane Number.....is the number given to a fuel as to it's ability to RESIST detonation under compression.
Basically, if you have an engine with about 8.8:1 CR...and normal ignition...then 91 RON fuel will be good.......BUT....if that 8.8:1 CR engine has an aggressive ignition tune, and an aggressive injection tune.....you will need 95/98 RON fuel to run with out pinging.
Also, on the other hand, if the engine has like 10:1 CR....your gunna need 95/98 fuel for better running...depending on cams and tunes, of course.
Cetane numbers are given to Diesel fuels......ability to detonate under compression.

Back in the day, car manufacturers offered up to 3 different compression ratio's far the same engine.....Holden was an avid doer of this...LC, MC & HC.....Low, Medium & High.
LC & MC could use the old Standard fuel ( 89 RON ), but the HC needed Super fuel ( about 97/98 RON ), remembering these were Leaded fuels.
If anybody here is old enough...remember the BP & Golden Fleece servo's had the big, wide mixer fuel bowser's....you could choose how much STD & Super fuel mixed together, from straight STD or Super, to a mix like 10% STD/90% Super to 90% STD/10% Super. If I remember....50/50, 60/40, 70/30, 80/20, 90/10....something like that......can't find any pics of them.

If a car was tuned to run on 98, would running it on 91 result in LEAN readings from o2 sensor?

i think it takes a bit more than one tank for the pcm/ecu, whatever u call the hoopty wha's these days, to work out the benifits, i've heard constantly changing fuels is like constantly changing oils... not good.

lol i use to use 91 in my EF XR8 i didnt give a F**k what fuel i used, even put that E10 in it and ran fine, no difference even with 98

No...not lean....just that the 91 catches fire when you don't want it to....pre-ignition.
ie: engine running properly..flame front moves at 20 feet per sec....burning.
engine running & pinging...flame front moves at 2,000 ft per sec....exploding....pinging.
Now, do you get it.
Ever seen a piston from an engine that has been running in poor tune to the point of pinging under light load.....looks like you found it out in a paddock...pits all over the top of the piston......the expolsions are removing bits of alloy from the piston.

That's because the 165kW V8 falcons all were tuned to run on 91 octane regular unleaded. AU's require premium because the heads have a smaller combustion chamber (resulting in a higher compression ratio) and a swirl ridge (resulting in a possible source of detonation).

Just be careful of any info off Wikipedia as it is a public database, edited by the public. While I am not refuting the claims touted on Wiki, you have to only use it as a means to get somewhere else like the references at the bottom. Using Wiki in assignments in Uni, you will lose marks or fail the subject if Wiki is used as a reference. If the Wiki doesnt take its info from a credible source - don't use it!

This said, you have the three types of fuel and most new cars will run on 91 fairly well. Realise when the driving becomes more spirited, the 91 will probably cause your car to run like a dog as it is not up to being put under pressure. I recently had big issues with independent suppliers and the quality of their fuel - they are too hit-and-miss with their quality and as a consequence I had to drain 50 litres of fuel from my car and dump most of it(kept 20 litres for the mower!!). I used Freedom 95(10 per cent ethanol)and it was right off(smell was like swamp gas when burnt and had other things in it that were way off the mark for normal petrol. Avoid this chain at all costs if you value your engine!!!!

I run nothing but 98 now and shell at that. BP lost my vote when they gave me s**t service and non-explination as to why their LPG went up by 15 cpl above everyone else for over 9 days. Must have been paying for their executives to go on a team group hug on a cruise ship or something.

One independant supplier where I live is notorious for having crap fuel - I got hit by it as well, and never went back.
I now stick to the 'name' brands, and whenever possible fill up at the major service centres, to make sure there the quality is an acceptable standard.

The local metro servo here on the central coast sell 95 with e10 and the 91 is normal 91