[Scoob]

I am fairly certain however sideloaded content does sync when you use the send to kindle service. There are two ways to use that service one is to simply email the epub to the kindle app and kindle. The email address can be found in the settings menu. The other method is to use the website send to kindle. Whichever you choose will automatically convert your epub to a kindle friendly format.

Make sure whispersync and wifi (or data on phone) is turned on both the kindle and on the kindle app and it will sync the bookmarks, notes, highlights, and last page read.