What's the effect of low viscosity in Heat transfer Oil? Such as effect to the tube temperature or Heat transfer value

What do you think, which one of the two has better heat transfer properties: water or honey? Lower viscous liquids with low VI have better mobility, and are easier to pump through a system from heated environments to chillers and back. For that reason alone, viscosity of transformer oils are limited to no greater than 12 cSt.

But The HTO Product has viscosity grade 30 ct and Esso put condemning limit of it is plus and minus 20%. And afterlong time operation oil viscosity drop down until below 20 cst
NB ;Product is EssoTherm 500.

Low viscosity on a heat transfer system with ISO 32 grade will be - top up with lower viscosity oil or, more likely thermal cracking of the oil due to overheating. Check the closed and open flash points. A difference of 20C or more will indicate cracking.
If the system has been in use for a long time you likely have a build up of deposits causing hot spots.