Which OAUTH token do I need to use to allow access to user IDs?

API
2

Technically none.

The users API requires no scopes and no Token at all

It is suggested you use an App Access Token

To get the higher rate limit of 800, but since you already have a Bot User Access Token, that token will suffice to get the higher rate limit and future proof you ready for

But the users API lets you look up 100 users in a single request.