[alanmcorcoran]

In the States, the vernacular for low octane misfires/pre-ignition/detonation/dieseling is "pinging" and "knocking." Of course, none of these sounds sound like the words we use for them, it's just the lexicon we've all agreed with. I don't think that's what your sound is, but just in case it is, it is typically caused by an engine that, for various reasons, requires a higher octane.

I experimented with various octanes and confirmed what Easy and IQ have posted here: In a standard GZ, you don't need any more than the cheap 89 stuff. I don't get any pinging or knocking with the lower octane. I have no personal experience with ethanol but I understand it has a higher "octane" rating and the 98 number you have is a result of a blend (I think pure ethanol is 116.)

Ironically, one of the side effect of ethanol is it is less efficient than "petrol" and it reduces gas mileage. Probably not so much that you notice, especially on a GZ, and you said you weren't worried about cost anyway.

I think your problem is your oil is too low. Most of the motorcycle tips I've read recommend you start and let the bike run for a few minutes before you take off. This is partly to make sure that the oil gets distributed around on all of the engine parts before you put them under additional stress.