Login from one Drupal instance on another Drupal instance I'm working on a project that has some unconventional requirements. 

Basically, I have two sites, each one with an individual Drupal installation and instance. The main site (I'll call it "master") has a lot of registered users, and the other one does not (I'll call it "slave").

I need the possibility of logging into the "master" site through the "slave" one. Like, the "slave" site would have a login form, that would connect to the "master" site and authenticate the user.

I've thought of using APIs like OAuth for this, but I didn't find any solid documentation or tutorial of how to do this with Drupal.

Observations: I can't make them multisites, they are two isolated Drupal instances. And it must work for Drupal 6.

Can anyone give me some directions?
Have a look at the Single Sign On module. It's only for Drupal 6, which is what you need, but it's not actively maintained. If it works as advertised, I think you are good to go. 

There is also the Bakery Single Sign-On System, but I don't think it fits your specs and requirements.