I know that it is an old thread.
Recently I read the Khyber's owner's manual, and the speed ranges for the gears are surprising. They are as follows:
low - -30 km/h (19 mph)
second - 10 - 50km/h (6-31 mph)
third - 20-90 km/h (12-56 mph)
fourth - 30 - 120 km/h (19-75 mph)
fifth - 35 km/h - (22 mph -)
According to the manual, you can change gears anywhere in these speed ranges.
So in theory, there should be no damage to the engine cruising at a very light throttle in 35 km/h (roughly 1000 rpm). The instantaneous fuel consumption would shoot through the roof too.