D Winsor comments on thermal expansion.
I only know something of this from fuel oil heater control.
It was my understanding that leaks were a possibility during shut down when changing between HFO and MDO because, at their very different temperatures, differential expansion would see flanges cool before the bolts (which is why the engine manufacturer would not insulate the flanges on the viscometer). If temperature or temperature control is a possible problem, the question to ask is if the fuel temperature is where it is expected to be? If it is too high, is this because there is a problem with the viscometer? (not sure which viscometer Wartsila use today).
PS Fuel temperature/viscosity and density/temperature spreadsheets are available at http://www.viscoanalyser.com/page8.html
which might be useful.