I used to read old Popular Mechanics magazines and I remember one article from the late 40's or early 50's where it was discussing motorcycle maintenance and it explicitly stated that every 5,000 - 6,000 miles you should rebuild your engine for best performance.

Of course, the engine was a two-cylinder that had maybe 25 parts to it, the rebuild process only needed a crescent wrench, a screwdriver, and some gasket making chemicals, and was so simple that 2 pictures told you all of the information you needed to succeed.