The Dropbox API does now have two different types of access tokens: "long-lived" and "short-lived". Long-lived access tokens don't expire by default, while short-lived access tokens expire after a short period of time (with an option to use "refresh tokens" for long-term access). Dropbox is moving to exclusively issuing new short-lived access tokens and no longer returning new long-lived access tokens.
The "sl." prefix indicates that that access token is short-lived and will only be valid for several hours. (Specifically, it's four hours, but you should get the actual expiration time from the API itself.) If you do need access for longer than that, you would need to request "offline" access in order to get a refresh token which can be used to get new short-lived access tokens as needed, or otherwise have the user occasionally re-authorize the app.
I recommend referring to the following resources for more information:
