Is your manual from the US or Canada? We use AKI here, not RON. In North America, we use 87 for the Prius (or "regular unleaded" fuel). 95 RON is the equivalent of 91 AKI in North America or "Premium Unleaded". Is your mechanic familiar with Toyota hybrids? The Prius has an unusually high compression ratio due to the Atkinson cycle. Normally a car on the Otto cycle with high compression would require higher octane fuel. (e.g. Mazda SkyActiv engines outside of North America)
There is also the BIG debate / controversy about 10% ethanol vs non-eth gas. On this side of the pond most non-eth gas is higher octane at the pump. Many subscribe to the non eth gas being better that the 10% ethanol stuff. Me too. But it doesn't stop me from filling up with the 10% stuff too, especially while on a road trip. The main issue with the 10% from what I've seen in tests is that the ethanol absorbs water whics helps to corrode stuff. Than there is the winter blends, and I don't know about anyone else, but I've been to gas stations in wintertime that smell like the home heating oil that still gets delivered to places that use oil burners as a heat source. Anyways, prius and most other cars these days have sealed gas lines that only get opened to the atmosphere when being refueled. Take your pick. Run your mileage calculations and use what you want. If there is a noticeable difference someone would have put the controversy to rest by now.
In North America “87” is regular, which the manufacturers recommendation. In New Zealand (worth noting) it appears “91” is the equivalent: Information about fuel | Fuel Quality Monitoring.
You're okay down to 90 RON, equivalent to 86 AKI. I don't know exactly what they stock there, but I'd be fully confident in a 90 or 91 RON product in a Prius.
In North America, the answer is easy. Regular Unleaded. Done. If you were in North America I would also confirm that you need another mechanic - and this still might be the case for you because there is a misconception (at least here in America) that higher octane gas is 'better.' We even call it 'premium' gas instead of the 'regular' stuff that 'lesser' cars use. An informative article from the kids over at C/D... https://www.caranddriver.com/features/a28565486/honda-cr-v-vs-bmw-m5-ford-f-150-dodge-charger/ ....Raising the octane rating (also known as the anti-knock index) doesn't change the energy content of a gallon of gasoline. A higher octane rating indicates greater resistance to knock, the early combustion of the fuel-air mixture that causes cylinder pressure to spike. When higher-octane fuel is flowing through its injectors, the engine controller can take advantage of the elevated knock threshold and dial in more aggressive timing and higher boost pressures to improve performance..... I suspect, though I will not assert that your NZ manual says to use 91 (RON.) A guide to petrol in New Zealand | AA New Zealand If so - do that, or at least ask your mechanic why specifically the higher octane fuel is....recommended? required? for your car. If he or she mentions higher cylinder compression, then you might note that performance cars also use engines with higher compressions and they almost ALWAYS recommend (or require) that you use the higher octane fuel. If their customers try to use the cheaper lower octane fuel then the engine's fuel management system adjusts for this and it simply makes a little less power. Priuses are manufactured to be fuel efficient, and I've never seen a factory manual recommending a higher than 'regular' or 'standard' fuel - but I haven't been to your nation. Also.... You live in NZ. It's a 'topographically interesting' place - and higher altitudes 'can' make for richer fuel-air mixes, reducing your octane requirement - although most of your citizens probably live on or near the coast. Your engine uses something like a mass air flow sensor to adjust for this. Bottom line: If your car is efficient, your fuel is good. The MPG readings in Priuses are notoriously inaccurate, but they ARE comparable. Gas is expensive and some people think that you should use as little of it as possible either because it IS expensive or because we need to be good stewards of our planet. High compression engines are equipped with 'knock' (predetonation) sensors that should notify you when your octane level is dangerously low well before any actual damage occurs. Run a few tanks of the various blends in YOUR location with YOUR car and see if YOU can tell a difference. If you cannot tell a difference (excepting for the cost) then there probably isn't one.....
