Diesel fuel does loose some of its viscosity when heated. Depending on how much you heat it, it also becomes more compressible.

NACA has some papers that discuss these things.

Viscosity of diesel engine fuel oil under pressure http://naca.larc.nasa.gov/reports/1929/naca-tn-315/
The last page has a temperature vs pressure graph on viscosity


Influence of fuel-oil temperature on the combustion in a prechamber compression-ignition engine http://naca.larc.nasa.gov/reports/1936/naca-tn-565/
This one touches on the compresibilty issue

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