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I have a Japanese imported 2005 Nissan Tiida 1.8L. Is the engine designed to run on 91 octane or 95 octane? I found the information in the owner’s manual (printed in Japanese) says unleaded regular petrol. But there’s no way for me to find out what unleaded regular petrol stands for.

I also have a Japanese imported 1999 Toyota Starlet 1.3L. I was told by the car dealer that I must use 95 or 98 as 91 is no good for the car engine. Is this advice correct?

Both of these cars can operate fine on 91. Regular fuel is the term used for 91, Super is 95.

Thanks so much for the information, that really eases my mind. Although I’ve topped up both cars with 95 in the past just to be on the safe side. If 91 is fine for both of my cars, it shouldn’t do any harm to the engine, am I right?

I heard some people say NZ 91 octane is rubbish, does this make any sense? That’s why I always top up 95 for my 1999 Toyota Starlet, and I never came across the question to check if the dealer actually provided me the correct information.

In other threads I found your advice to other people “If the vehicle is designed to run on 91, you're usually best to stick to that as there's generally no benefit in running a more expensive, higher octane fuel.”

In this case, other than in performance and improved fuel consumption reason, what petrol type should I better stick to? Or it doesn’t matter.

There is absolutely nothing wrong with New Zealand 91 octane fuel, or any other NZ fuel.
If your car is designed to run on 91, there's generally no advantage running on 95. You will get slightly better fuel consumption on 95, but the advantage doesn't stack up when compared to the added cost of 95.
There's no harm in using 95, but no benefit either. So, it doesn't really matter which you use, but 91 will work out cheaper.