Not to be vague here but the best place to check that info is the vehicle owners manual (if you still have them). It gives the minimum recommended octane levels as well as any cautions the manufacturer thinks you should be aware of. If all else fails, call your local dealer and speak to the service manager.
I do remember hearing recommendations that I should avoid ethanol laced fuels because the ethanol attacks the o-rings in the fuel system thereby creating system failures. I've also read that later year models had ethanol tolerant o-rings in them but I don't know when that occured. My 4Runner is also a 98 and I do avoid fuel containing ethanol whenever I can; that being said, I don't think occasional use will do any catastrophic harm to your vehicles.
