Originally Posted by Tazojla

Just tried a 16GB USB stick and it works, just a case of transferring .MP3 files into it and plugging it into the USB of the car.

Best way I found is to upload song data e.g Artist, Album Artwork etc through iTunes and create a playlist of your desired songs and copy them over by dragging and dropping them into the USB sticks window. Once the download has completed you can just plug it into the car and then go into Entertainment and find your USB stick (Will be under the name of your USB stick) and the music is there.
I couldn't figure out a way to make playlists so I just changed the genre so it's kinda like a playlist (Image attached)