I have a 1985 W123 230E, UK car just imported into the US last week.

So, this is for all of you far wiser and knowledgeable than me. What is the best American gas for my beloved W123?

At the end of the summer, the UK switched to E10 petrol (10% ethanol) at 95 octane. Car owners with cars pre-dating 2002 were encouraged to use E5 at 97 octane instead. I inadvertently filled up my W123 with E10 and did notice a difference; rougher idle, a little poorer fuel consumption but maybe I was being paranoid.

Fast forward to this past week. The car arrived in the US with 1/4 tank of E5. I filled it up with Marine Gas and, again, maybe I am being paranoid, the car feels quite a bit different. Higher idle, much perkier from a stop up to about 40, a little rougher idle when warm and sitting at a light but smooth at 60+

Could any of those perceived issues be related to the sea voyage?

The tank is back down to a 1/4....what would be the best option to fill the car up and keep her running happily?

NB:
Before leaving the UK, the car was fully serviced with new plugs etc and the injection system gone over for leaks or issues, fuel filter replaced but I am noticing a bit of a petrol smell that I had not smelled before but, bearing in mind these old 123s are a bit stinky!

Many thanks for your advice. I love this old car and rely on her, she's never let me down.

Understand first that there is no right answer to this question: everyone will just have an opinion.

That being said, I would run a clean-rated (Chevron, Shell, Costco) premium gas and never look back. If it knocked still, I could throw in some octane booster.

Then I would shift my thinking to something like, I wonder how the leaves will change this fall, or do I have the best plans for Thanksgiving?

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Prost!

I would run unleaded premium with no ethanol.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

First, understand that the UK and Europe use the RON rating method that yields 4-5 points more than the USA's PON.

What's the compression ratio or if you have an owner's manual what's the RON recommendation?

There is no "best gas" in the USA. It often comes from independent refiners and the only difference between brands is the additive package. I recall one petroleum engineer recommending switching brands frequently to get whatever benefits each additive package offers above the others, if any.

The bottom line is to use any name brand gasoline from a station that pumps a lot, which is most nowadays. High inventory turnover avoids any significant water buildup.

I use ARCO because it is usually less expensive that the other major brands and there are several stations near me.

I've never had any fuel system issues that can be blamed on gasoline on any of my cars, and the oldest is a 1963 Corvette SWC.