A quick and easy substitute is gear oil and chassis grease in a 50/50 mix, just make sure what ever you use is soupy. The worm gear and roller balls need to oily wet at all times and if the lube is too thick it can't flow into the ball nut. Penrite and Dynolite both make proper steering box lube if thats what you want, but the corn head grease will work just as well.
