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