Because of the mechanical speed control on KitchenAid mixers, it is not unusual for there to be nearly imperceptible differences between adjacent speeds and does not necessarily indicate the need for a rebuild or repair if the mixer is running well otherwise.
You can download a manual that includes the most common mixers from the 1980’s and 1990’s at The Mending Shed.
The manual for 4.5 quart mixers will include instructions for a 5 quart lift bowl.
If your mixer is a K45 or K5-A from the 1970’s, overhaul is complicated a bit because parts are not generally available, but the procedure is generally the same as outlined in the service manual.
Repairing and overhauling a KtichenAid takes some mechanical experience and a couple of specialized tools, but is fairly straightforward. There are also videos on You Tube that will help you find your way.