I'll find the info. I'll concentrate on German sites, they're the best for these films I suppose.
I've planned to discuss the most important Winnetou & Old Shatterhand movies here in the near future
People can add (or correct) info, and eventually we can turn these reviews into official ones
The films were important for the European production of western movies in the 60s
What you say about the 3 million/12 months and the Goldene Leinwand is of course true, films can have a continuing story of succes. But this would mean that the film was not only an instant succes, but also a real sleeper: as far as I know, most visitors for a movie go to the cinema when the film is first released. I remember that some of the Winnetou movies were shown in cinemas in Holland and Belgium on a (more or less) regular base during the 60s, but usually in so-called 'matinees' : afternoon showings for children.