It is very well known that the higher the viscosity the higher the resistance offered. But why do we slip more on oil as compared with water even if oil has higher viscosity than water?
Because water "squishes out", allowing the sole of your show (which has treads for that reason) to contact the ground directly, and experience friction. Shoes with smooth flat soles are quite slippery on water, if the floor is also smooth and flat.
Oil, because it is more viscous, does not "squish out".