So long as you replace the diff putty every 3-4 full race sessions you're good. The only time the diff putty leaks out of there is too much diff movement, hence the reason I add an additional diff shim. The putty I'm referring to is thick enough you can roll into small balls. The instructions show rolling into 3x 10mm balls and placing it between the planetary gears. I did this the first time around the diff came too loose after a few runs. I now pack the entire diff, just enough so there is little diff action. Helps to pull the car around the corners. Not sure what blu-tac is? Its almost full lock, but there is some diff action with some efforts