actually the Synchronization options menu is under the split screen dropdown menu

I think it's completely out of place and does not belong to that place...
I say that because it happens frequently to me to forget where those commands are and spending time to find them and most of the time the split screen submenu is the last place I go looking at.

I'd rather think the Sync option deserves it's own toolbar button and dropdown menu

a logical position would be before "Browse series" buttom.

the main toolbar icon would be assigned to toggle on/off (actually is the Auto Sync item) while the dropdown would be the current "slice position", "zoom & pan" and "Window" items

regarding the icon I suggest a "chain icon" ... try doing a query on Google images and you'll find many of those

I'm not sure if a separate toolbar icon is a good idea.
We avoid clutter & complication as much as possible.
We have in mind a chain icon somewhere inside the viewport, actually.

I don't think moving a very used feature in the toolbar will cause much clutter...
that's a very used feature and I think much more used than the "cine" which is already in the toolbar.

if I could customize the toolbar I'd move "Cine" under the "browse series" as a submenu item and move "Sync" from the "split screen" submenu to the toolbar itself

moreover once "manual sync" will be available ( see http://www.radiantviewer.com/dicom-viewer-forum/manual-series-synchronisation/377 ) another submenu item will be added to it

Lots of RadiAnt users open ultrasound/angiography cine loops, they'd probably prefer the Cine button much more than the Synch button ;-)
Perhaps some toolbar customization would be a compromise here.

I think that the radiant toolbar has still enough place to have both Cine and Sync icons, so it's not necessary to remove Cine to make room for Sync

obviously toolbar customization would please anyone

just a reminder about the "chain" icon for the sync option

You keep fighting for it, Tommy ;-)

... and I never surrender :-)