For those of you that have never seen this clip before, skip to 1:20 for the sled action (before that is all the brother talking, he's kind of loopy).
So how much does greasing up an object truly effect friction?
Between two metals (lets use two hunks of aluminum for example), the coefficient of friction is roughly 1.05 to 1.35. When greased however, mu drops down to .3, which is anywhere from a third to a fourth of the original coefficient. The same goes for the coefficient of friction between snow and Clark's steel sled. The coefficient of friction between snow and steel is roughly .1. The Griswolds were sledding at night, so if the snow turned to ice the coefficient would be remarkably lower: 0.015. Add some canola or olive oil spray to the mix, and friction would be extremely small.
In other words, next time you break out the toboggan for some serious sled races, make sure to pack the pam!
P.S. I didn't pull those numbers out of a hat, my main source is http://www.engineeringtoolbox.com/friction-coefficients-d_778.html. Thanks, google!