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In the states, 100-octane was fairly common in the 60s and used in high-compression engines.
In my opinion it would be a nice option to have in the happy years just before the oil crisis. Also even after the ban of leaded gasoline it could be used in racing aplications.
What about other countries? Was 100-octane fuel also used there?

Considering how little difference in engine performance there is between 98 and 100, I don't think it's something that is required but adds a bit of unnecessary complexity. The good thing about the "early" years is that they are pretty easy to navigate in car design, because there just aren't that many options - which is great for a learning and complexity curve.

Well if it was a US specific thing I can understand it.

There was 100, 101, and sometimes 102 octane fuels in the US in the 60's/early 70's. But these weren't RON rated, these were AKI rated, so there is a big difference between the 93.1 octane available in game. One thing to understand though is that these ones weren't using the R+M/2 rating system, so in reality they were closer to 95-96.

However, they were widely available. Hess had 101 octane almost everywhere, Sunoco 260 had 102 octane, both leaded and available past 1975, since it was only new cars that required unleaded. Not much past 75, since that was the death toll for these fuels, but they were around for sure. Ford Boss 429's had 100 octane as basically a requirement. Nowadays, Sunoco still offers 110 octane fuel, and not too uncommonly, it's just VERY expensive.

is it even needed?

sure, more octane is always good, but come on

edit:

i would love to see devmeth

Devmeth needs to be a thing in sandbox

100 RON fuel was also widely available in Great Britain from the Sixties until the Eighties. During this period leaded Petroleum Spirit was graded according to a star system where:
1-star = 89 RON, 2-Star = 92 RON, 3-Star = 95 RON, 4-Star = 98 RON, 5-Star = 100/101 RON

The average car would run on 3-Star. 5-Star was used by sporty or luxury cars with a high compression ratio. The most commonly found, and the last to survive (until 2000), was 4-star. A lead-free version of 4-Star (called Lead Replacement Petrol or LRP) was available for a further couple of years in order to serve the dwindling number of cars without hardened valve seats. Thereafter the classic car community, as the only market with a need for leaded fuel, has generally resorted to using fuel additives. 100 RON petrol was termed "Aviation Spirit" during the war as it was reserved for use in warplanes.

Given the wide range of fuels available worldwide, would it be possible for Automation to use the term US Regular for 91 RON, Euro Regular for 95 RON & Premium for 98 RON unleaded fuel?

This

In fact, even though it is an option in game, the US fuel pumps don't even offer 90 AKI (I've never seen it). There are 87, 89, 91, 93, sometimes 95, and sometimes 110.

It's true that the octane ratings used in the game are actually not being offered at fuel pumps around the world with the same designations, etc. It doesn't matter though, whatsoever. More fuel types and region dependent octane ratings beyond the fuel types we have would make the game really complicated to play in the tycoon part. It adds a lot of complexity for little to no gameplay benefit. So no, we are not going to make fuel types more realistic or complete.

Also consider that octane listings within the US also vary +/- 1 octane somewhat from state to state, so it's not even the same throughout the country.

I agree with Killrob that it's close enough.

It varies due to altitude, the higher the altitude the lower the required octane. You can get 85 in Elko, NV but only 87 in Reno, NV.

I, for one, can tell you of at least one station in Reno that sells 85 octane, and another in Sparks, as well.

(Gasp!)