This is all very interesting. Especially a V6 Ecotec getting any benefit from anything other than Lucifer's brew (91!). I'm not going to bother putting anything but 91 in mine because it's 20 years old, done 210,000km and I'm hardly getting it warm driving to and from work every day. No point.
However, we do have a 2015 X-Trail outside that has been subjected to a single tank of 91, and it didn't like it one bit. I run it on 95 now, but danged if it's entirely happy with the arrangement. It's only got 41,000km on it, 38,000 of them on a steady diet of Eneos hi octane with occasional interlopers whilst traveling.
(Sharp eyed viewers will note that Eneos isn't exactly common around these parts, and some also know that it's the connoisseurs choice...)
If you haven't worked it out (or don't want to, which is also fine) the family bus isn't from around these parts either, so has a 2.0 direct injection engine and pretty high compression ratio. It's at least 2 generations newer than the 2.5 they stick in the local models, and is supposed to tolerate 90 octane but it does not at all like it, making itself known by pinging pretty hard under moderate load and requiring significantly more throttle to accomplish modest motion. Direct injection is supposed to limit pinging, and it kind of does but there's no getting around low engine speed, heavy load and high cylinder pressure. There's only so much timing on the spark and fuel, and to make up for the shortfall you've got a add a lot more fuel.
In short, it burns about 6% more fuel on regular than it does on high octane, which was enough to just run it on high octane for the $$$ savings. Everything else (and not much, it's a family bus not a sports car) was a bonus.
If here was like where we got the red bus, and high octane was only 10c/l more, it'd be sucking down 98. The local cheap place, 95 is 13c more, 98 is 20c more, which all things considered is quite obscene. Heck, let's be honest the whole "owning, running and fueling a car" here is obnoxious enough, so don't let anyone tell you "owning, running and fueling a car in Japan" is terrifying, here is worse* (but not by much, let's not get carried away).
Stu.
*for example; The red bus we bought cost about the same here, $ for yen. There are features on our 5 year old that still aren't available on the newest model of it here for any money. And we had to wait 6 weeks for them to build it, to order. I wish I'd updated it before we bought it over, just to rub it in a little more. But we did have a second car there that cost us $2,500 to buy, was 9 years old at the time, cost $200 a year for full rego (Kei car!) and while it wasn't perfect, it was in very good shape for it's age. Flip to here, you can't find a car half as clean and sweet for double the money and age, and running it would automatically cost 4x the rego. Fuel would be slightly cheaper, but over the course of a year for a true 'second car', I know we were a lot better off over there. And because you could thrash the thing and still use FA fuel, it was awesome, so was cheaper to fuel as well.
My only regret was having to buy family cars while I was there, because family. Finally get the opportunity to buy what I want for me, and Covid screwed that up, backed me into a corner and I had to settle for something that cost me twice as much and has chewed up 2x the cost in parts the little car did. And I still didn't get what I wanted by a long way.
The way things have gone, I'll have to build what I want. The only question is how deep to go on build...