Really sorry for the belated reply.
Long story short, mpeg4 or h264 video and aac audio inside an mp4 container should be fine, provided that you select “Original” when you add the item to sync. You’ll also need to make sure that there is no selected subtitle stream, as having one would lead to conversion as well.
If you want to see the exact cause why a synced video needed to be transcoded then you can check your PMS logs. This, for instance, is what I see when I add an hevc video to sync:
Sep 30, 2016 18:37:43.692 [0x70000021d000] DEBUG - MDE: analyzing media item 12347
Sep 30, 2016 18:37:43.692 [0x70000021d000] DEBUG - MDE: AAC / h265 (mkv) (2015): no direct play video profile exists for http/mkv/hevc
Sep 30, 2016 18:37:43.692 [0x70000021d000] DEBUG - MDE: AAC / h265 (mkv) (2015): no direct play video profile exists for http/mkv/hevc/aac
Sep 30, 2016 18:37:43.692 [0x70000021d000] DEBUG - MDE: AAC / h265 (mkv) (2015): no remuxable profile found, so video stream will be transcoded
And this is what I see when I add a h264/mp4 video with a selected SRT subtitle stream:
Sep 30, 2016 18:41:13.145 [0x7000003a6000] DEBUG - MDE: Birdman or (The Unexpected Virtue of Ignorance) (2014): selected sidecar subtitle stream cannot be direct played
Sep 30, 2016 18:41:13.145 [0x7000003a6000] DEBUG - MDE: Birdman or (The Unexpected Virtue of Ignorance) (2014): selected subtitle cannot be converted to a compatible format, burning into video stream
Sep 30, 2016 18:41:13.145 [0x7000003a6000] DEBUG - MDE: Birdman or (The Unexpected Virtue of Ignorance) (2014): avoiding video remux due to burned subtitle stream
Sep 30, 2016 18:41:13.145 [0x7000003a6000] DEBUG - MDE: Birdman or (The Unexpected Virtue of Ignorance) (2014): no remuxable profile found, so video stream will be transcoded
And finally the same video after I’ve unselected the subtitle stream:
Sep 30, 2016 18:43:52.507 [0x700000ee8000] DEBUG - MDE: Birdman or (The Unexpected Virtue of Ignorance) (2014): DirectPlay
We’re aware that better subtitle support would be desirable, and also that allowing hevc files to be synced without conversion on devices that can DP that format would be a more than welcome addition. But for the time being and in the short term the limitations I’ve described will exist.