91 octane as read in the Owners Manual and do not use any octane boosters, they don't work anyway most are alcohol based and actually lower the octane rating of the fuel. If I could get higher thru the pumps in our state, I'd run higher. Do not use regular or mid grade in the N. The N has a 9:5.1 compression ratio and runs over 20 lbs. boost. A similar Theta 2.0 engine without a turbo can run regular. However ,with this kind of boost and it requires, premium top tier fuel. Regular grade fuels will cause pinging and detonation, which is very bad for any engine.
The salesman, nor your dealer has any idea or knows very little about the N. Read the manual and stick to what it recommends. May try some VP Racing fuel also..
Blessings and Peace