I have one simple question. I have bought Kitchen Aid classic. When I have tried to run it, I didn't notice any difference in speed between level 8 and 10. Is it normal or it is broken?
The difference between 8 and 10 is small on a Kitchen Aid, but measurable in most cases. I tell the difference by pitch, which gets slightly higher on 10 than 8. However, if you are mixing a thick mixture there may be almost no difference in speed or pitch at all.
A simple way to tell for sure is to run the machine with no attachments. Use a stopwatch and count the number of rotations in a 30 second period on speed 8 and then on speed 10. There will be more rotations on speed 10 than 8 if the mixer is working properly.
If you aren't getting a speed difference between 8 and 10 then there is some sort of problem with it. A common issue with old mixers is the grease inside breaks down and thickens, there could also be a problem with the motor or the variable resistor which controls the speed. None of those are impossible to fix but it's always a good idea to start with re-greasing.