I think basicly the 400 bandit is like the across 250's
The minister of war, ( she whom must be obeyed) and I ran 2 across's for courier work, going back a few years.
Mate they are unbreakable, in the middle of summer down under working the CBD, with the temp gauge always at 3/4, being flogged.
Both units had well over 60 thousand miles up when we sold them off.
Both ran really well when sold, and in fact the oldest one still pulled a genuine 110 MPH. Both engines had never been pulled down, valve adjustment only and some carby tinkering only. One thing I did do is, install a manual switch to turn the fans on earlier, than the auto system did. It seemed easier to keep them from getting too hot than trying to cool them down, especially running around the city block all day.
The other is use good quailty oil, during the hotter months and depending on how many miles the engine done, I would also consider using the next weight heavier oil.