If you run at wide open throttle, 91 is the way to go. If you are like me and only rarely open up my R, then 89 works quite well.
I used to use 91 all of the time, except for Interstate trips where boost is not used. I found the gas mileage to be the same for 89 and 91. The one time I filled the tank with 87 octane, my gas mileage suffered a lot so I guess the knock sensors were working overtime to enrich the mixture and retard the timing.

With 89 octane, the engine will not get up to 6,000 rpm due to knock sensors.

Here in the USA, it is still only 10cents more for each grade which translates into $1.80 for the 18 gallon tank. Not much of a savings! I don't know what happens with the per litre prices for each grade.

Klaus
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I still miss my 164