The effect of fuel viscosity on diesel spray droplet size and velocity is investigated in a high-pressure chamber using laser diagnostics. Particle image velocimetry (PTV) is used for the investigation of droplet velocities. Shadowgraphy employing a long-distance microscope is used for determining droplet sizes and for imaging the spray form near the nozzle exit. Double-frame microscopic shadowgraph images are also used for droplet velocity validations using particle tracking velocimetry (PTV) and micro PIV. Velocity measurements from PTV and PIV analysis of microscopic shadowgraphy measurements provide a favorable comparison with PIV results using laser sheet illumination. Two fuels of different viscosity are investigated. One is standard automotive diesel fuel. The other is a mixture of 75% canola oil and 25% automotive diesel. The mean spray penetration and angle are similar for the two fuels. Measured droplet sizes and velocities increase significantly with increased fuel viscosity. Spray density limits the axial domain that can be imaged for droplet size and velocity measurements. For light sheet PIV, diesel fuel, having the smaller droplets and thus higher optical density, is the most limited. For 75% canola fuel, larger droplets are formed due to the higher fuel viscosity, providing clearer images for the PIV algorithm. Microscopic shadowgraph images near the nozzle exit show the existence of Mach waves and the growth of shear layer instabilities. The origins of shot-to-shot variability and droplet clustering are illustrated and discussed.