Tape a novelty laser pointer facing down to the freehub of one of the front wheels. Print scale markings for about 5 meters using a drafter (at 1:1 ratio of course). Lay it down on a flat stretch. Align laser pointer to the printed scale. You should be able to calibrate it within 5 trials.
Please refer to the temperature dependence of liquid viscosity on wikipedia:
http://en.wikipedia.org/wiki/Temperature_dependence_of_liquid_viscosity
Plug in the already available data (40°C) to extrapolate required data (20°C)
You might want to find a super flat stretch of 30 meters (length of truck + 5 meters approx) and use the above mentioned laser/ultrasonic distance meter. Rinse and repeat