well, if you don't, your diff will probably seem like normal, but it probably wont be long when you will need a rebuild, i have all metal gears, and already spent 140.00 on the os .18 engine, but the grease is only 2.39, it may be best to buy some, or at least something close to it, but if you run it, id go to the hobby shop and get some as soon as i can, or spend more money and your car teared apart, like me, i can give you some, i have 3 bottles, but the 1 day shipping costs are horrrible!!!