So just to keep you'll up to date, am now on my second tank with Esso 89 instead of the usual #91 and thus far no more misfire codes, stumbles somwhatnstill on cold mornings but looks like my engine likes the lower octane better, will take the next step down to 87 on my next fill up.

Told you 😀.. glad your seeing improvements. It's ok to be running lower 87/89 in the cold winter months, every now and again throw some BMW fuel injector cleaner in it as well. Good luck and keep us posted!

UPDATE.

Been a whole month now on mostly octane 89 and no misfires to speak of, however it has been an unusual and rather mild February in my neck of the woods....

I know of no science that would explain how lower octane fuel might/will solve a stumbling problem making me a skeptic looking for the logic.

The basics of octane differences are that lower octane in a high compression engine where the manual calls for high octane will likely lose performance if the timing adjusts or spark knock if the engine does not have the capability or range to do so. Higher octane in a low compression engine calling for low octane will have have little effect on anything making it a waste of money for the extra cost. Stumbling cure or cause is not a part of either change.

The only long shot exceptions that I can think of is a change in the source of fuel such as trash gas to name brand gas, changing to a fuel that contains a fuel system cleaner, you are in a state where they have different additives for summer than winter that your engine doesn't like or, perhaps, something else that is causing the stumbling that using lower octane fuel is masking.

What vehicle are you referencing and what octane is recommended by the manual? What do you mean by "mostly" 89 octane? When in this scenario did you add injector cleaner? Were you always running Esso gasoline? How long had you had the stumbling issue? Have you done anything other than change octane shortly before or during this timeframe?

I'm a tech at a BMW dealership in the Chicago land area and all I know is we've had about 2-3 cases with these symptoms on this engine and changing to a lower octane fuel in a lower climate area has cured the problem on those vehicles. There were many different parts replaced on the same vehicles which didn't fix the problem but once running the lower grade fuel, just like that the problem was no more.

Cmyachtie - glad to hear your vehicle seems to be running well!

I am just as puzzled as you are ,having suffered from these random misfires sinceI got this car for 5 yrs now and only in winter, always run Esso 91 octane which is recommended by BMW for the 545i N62 engine(Esso here in my neck of the woods is my best choice available and have run this in all my previous BMW's for over 15 yrs now)
As far as lack of performance goes, only thing I have noticed is a delay in response when flooring the gas pedal, driving wise nothing noticeable but maybe better mileage (is still early days on that observation).
My last four fill ups have been one at 87 and three at 89 so there was a mix there when switch occurred, but now it would be straight 89 for the last two weeks or so.

Kudos to fasteddie for the suggestion, much appreciated!!!

If you're happy that's all that matters.

+1....LOL

To bad I can't test the lower octane fuel due to it is not sold in here. The lowest is 95(european standard which is equal to 91 in USA). I have tried higher octane, but that doesn't make any difference.

Now when the spring is coming, I don't have much of the problem anymore. During the days, the temperature is at least +5C and around 0C at nights. Starting the car in the morning is much better than in winter. Maybe a small rougness, but I haven't seen misfires for a long time now.

It is very interesting how a change of 2-5 degrees in temperature can completely change the cold engine behaviour.

Finally did run a full tank straight 87 and find that my sweet spot appears to be 89 as I still encountered two bad starts with the 87. I realize this is not the best scientific conclusion as weather was quite up and down as well during this period. It also seems that the bad starts are also affected by how long it's been since I ran car, parked for 3 days and nights was worst start I had in a long time with 87.