Shell vs Mobil vs Chevron vs ARCO/Costco/76 etc Which gas is “best” or clearly better?

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I live in Vegas. There are no real fuel stations. 91 octane is “premium” here. Long story short….I just bought a 2018 CTS-V, 4,800 miles. 6.2L supercharged. It’s stock. Owners manual says “recommend using 93 octane”…..”if no 93, use 91 under loss off mpg and loss of performance…..may cause engine damage”.
I have 2 choices really…
1. Use 91 Octane. If that’s the case, I ask which is better because some cars act better on certain fuels. I had a Syclone that would ping on 93 octane in Illinois but not 93 Mobil. I’m thinking Shell 91 or Mobil 91?
2. Arco has 100 octane. Is it ethanol free or 10% ethanol? My tank is 19 gallons and I’m thinking use arco 100 for 4 gallons and Shell/Mobil 91 for the rest.
Arco pics attached as examples.
 

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Around this part of the country, Shell has the best additives. Exxon and the Holiday stations also have good additive packages to make them top of line gasoline. Costco and Cenex also have Top Teir gasoline, For diesel fuel, Cenex has the best fuel bar none.
 
100 octane unleaded is always oxygenated. The best we can do with a conventional "gasoline" is 95 to 96 (R+M)/2 octane without lead or oxygenates. Yes, there is always the downright amazing Toluene or Zylene as an octane booster, but that comes with operational difficulties in cold weather and low load conditions.

It is very likely that the 100 octane is a 95 octane base fuel with ethanol.

In a boosted engine, in hot weather, don't be afraid to add 10% Toluene. It will vaporize nicely under those conditions and provide epic octane. The racers getting 1500HP from 1.5L F1 engines heated the fuel to nearly 300 degrees to make it work. After I learned this in the 1980's, I used Toluene in my boosted vehicles. Running 33 pounds of boost regularly on pump fuel and Toluene
 
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In the aviation world, we still use 100LL (low lead) because until recently there was no viable 100 octane non-oxygenated unleaded. GAMI (company) produces a viable 100UL, and to get the necessary performance, I believe they have to use up to 6% of an unusual amine additive that typically comes from the dye industry in Germany, and was selling for $250 per L.

There are a few other candidates for 100 octane non oxygenated unleaded fuels. One interesting candidate uses trimethylbenzene and isopentane mixed in the correct ratio to achieve the octane. Good luck starting an engine in 40 degree weather.... Maybe some additives will help...
 
I think the octane rating is tightly controlled by the government went it leaves the storage facility. However Octane degrades over time, so you likely want whichever station cycles fuel fastest. Problem is that's difficult to tell because the highest Octane is only purchased by a very small number of people.

I think your going to have to try them both and figure it out.

You may also wish to sample both for how much ethanol is in them. Its easy to do - you can buy a kit or just get your own graduated cylinder and search on youtube.
 
I live in Vegas. There are no real fuel stations. 91 octane is “premium” here. Long story short….I just bought a 2018 CTS-V, 4,800 miles. 6.2L supercharged. It’s stock. Owners manual says “recommend using 93 octane”…..”if no 93, use 91 under loss off mpg and loss of performance…..may cause engine damage”.
I have 2 choices really…
1. Use 91 Octane. If that’s the case, I ask which is better because some cars act better on certain fuels. I had a Syclone that would ping on 93 octane in Illinois but not 93 Mobil. I’m thinking Shell 91 or Mobil 91?
2. Arco has 100 octane. Is it ethanol free or 10% ethanol? My tank is 19 gallons and I’m thinking use arco 100 for 4 gallons and Shell/Mobil 91 for the rest.
Arco pics attached as examples.
Does Vegas have E85 available? If you think your V is crazy fast now, get a good ethanol tune and HANG ON… pump ethanol usually has an octane around 105, so in addition to giving you some protection there, ethanol has a higher latent heat of vaporization means it cools the intake charge much cooler than gasoline.

A simple 30% ethanol tune took my truck from ~320rwhp to a hair over 500rwhp, and it’s readily available anywhere here. It does cause about a 15% loss in mileage, but to me it’s worth it! Good luck 👍🏻

 
Does Vegas have E85 available? If you think your V is crazy fast now, get a good ethanol tune and HANG ON… pump ethanol usually has an octane around 105, so in addition to giving you some protection there, ethanol has a higher latent heat of vaporization means it cools the intake charge much cooler than gasoline.

A simple 30% ethanol tune took my truck from ~320rwhp to a hair over 500rwhp, and it’s readily available anywhere here. It does cause about a 15% loss in mileage, but to me it’s worth it! Good luck 👍🏻

There is E85 here. That was choice 3 but last choice. I want to add a lid and pulley, then tune it. And when tuning, either do E85 or meth kit.
 
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