Seller id is sapporo10 but doesn't appear to be selling any for e30 318iS at the moment.
Pre-chip I almost exclusively ran my car on 98RON fuel; regular has been going around $1.20-1.35/L and the cost of 98RON is always $0.10 more than whatever regular unleaded costs. This means that its always less than 10% extra but I noticed a decrease in fuel consumption of more than 10%.
However recently I didn't have much cash on me and I filled up with regular unleaded (knowing that the "octane rating" was going to be "minimum 91"). The ProFormance chip stated to use fuel with 91 or higher octane rating.
I noticed the car knocked at low revs under load so I researched octane ratings and found that the fuel I'm using is rated waaaay too low for the tuning of the chip.
The different octane ratings between US and Australia are that the US uses the AKI which is RON+MON/2 and Australia uses just the RON. MON is "generally 8 to 10 less than RON" so 98+88/2 = 93/94 US octane rating.
I didn't know this at the time and hope that there hasn't been any damage.
I'd say if you make sure you NEVER use fuel less than the specified rating and you should be okay. I'd heard all this garbage about US fuel being poorer quality than Australian fuel, obviously propagated by people who didn't realise the difference in octane rating systems between the companies which is why it never even crossed my mind that this might be the wrong fuel to put in.
I thought the fuel I was putting was a-okay; I was wrong

I'm driving it really carefully at the moment, keeping revs higher to keep cylinder pressure down but I still get a knock every now and then. Is this going to cause major damage?