I had encountered a similar situation as well for my F30.
The problem was my reserved reservoir connectors have worn out and the water is depleting gradually. I had to top up like every 3-4 days. (Frustrated and ).
After rectifying this problem, I noticed a few significant changes. ()
1. my temp reading stays at between 110++ degC while travelling. Never hit 120deg C. It does goes lower during cooler weather as low as 100 DegC ++ at times.
2. It takes a much longer time for the temperature to climb to 120 degC while travelling.
3. Air con appears to be cooler. (24 degC)
My ride is into its 6yrs and mileage is super high at ard 157, 000km.