Heating will lower the viscosity significantly, if perhaps you had a heated fuel system and filtered the oil very well....

Diesel has a kinematic viscosity of 1.3 to 4.5 at 40C GTS 32 has a KV or 30 @ 40C and 5.2 @100C There is a chart showing the relation of temp to viscosity of common oils at http://www.frybrid.com/forum/showthread.php?t=7687 but it is important to remember that there are other factors such as calorific value of the fuel and how the fuel cracks in the combustion chamber. Viscosity only relates to the level of atomization and the lubrication provided, some fuels do not require the same level of intimate contact with 02 to combust well. It is also worth noting that the viscosity of today's ULSD is very low and is likely damaging your older engine. Many in the marine world are now blending 5% motor oil with thier ULSD to restore lost lubricity - this of course raises the viscosity. Lube oil and exhaust gas analysis are the only real indicators.

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1971 Porsche 911 Targa RSR rep.
1968 BMW R60/2
1981 BMW R80GS-PD, dual plugs, 1000cc jugs, 10 gal "Gaston" tank.
1982 BMW R80GS-PD, duplicate of above.
1988 Neoplan/Mercedes 40' Bus
2002 SLK32 AMG
2013 Smart Electric