Recently changed the gearbox oil on my 1.0SE Lupo due to see if it helped with crunchy gear changes. Well it did, the gearbox is now smooth as silk at normal town speeds but has developed a new problem. If I have been driving on the motorway for a while (say 30 mins +), when I leave the motorway and have to start changing gear regularly again, the gear stick movement is very stiff and it takes a lot of effort to change gear. This problem ONLY occurs after the car has been in 5th for a while at high RPM. Anyone have any ideas on what might be causing this? I have no idea where to start looking... Thanks in advance