You’ll need to remind us of your setup @EcoBaker.
Do you have a buffer tank**?
Do your radiators have TRVs? (If so, could they all be closing for some reason?)
I’m pretty certain that the water viscosity isn’t the problem - your water temperature difference between heating and cooling is probably only 20-25degC (viscosity might be 20% more, and density maybe 5% more, but that shouldn’t kill the hydraulics).
**Edit: If so does your arrangement look like this?
and if you have a 3rd party roomstat, is your arrangement consistent with this?
