Question...

What is the last production Mercedes that used "regular gas"
as in 89 octane?

When introduced to the USA for 1984 the 190E 2.3 was a "regular fuel" (87 PON) engine. The new for '87 2.6 was recommended 91 PON, but they will run without detonation on 87 in cool to cold weather.

The 2.3 disappeared from the USA model line up for a couple of years in the late eighties, I recall. It appeared again in the early nineties, but with higher compression that required 91 PON.

Maybe others can fill in the details.

Duke

my 84 500 sec took regular.

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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.

sure enough I looked it up on the g'ment MPG website and it says
"regular gas"

cool.