I have a 540i and posted a similar question last year.
My take on it is stick with 95 and save your money. Whilst 99 may give some minimal increase in power I really only think you would notice this under racing conditions.
I have tried 95 and 97 and can't tell any difference.
Higher octane fuel may give a very small benefit in fuel consumption but the extra cost of the fuel wipes it out.
The premium fuel may offer some extra cleaning benefits so perhaps worth a try from time to time as regards that but in summary my view is to stick with 95.
I think there is a lot of imagination taking hold when evaluating these fuels , any comparison would have to be undertaken under laboratory conditions to be of any meaning.
I now await a raft of follow ups with different views!
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