This is a 40cc bottle of Kyosho Silicone differential oil. The thicker the oil, the less your differential will "diff out" in the corners, where the two tires will turn at different speeds. Changing the diff oil in your diffs is one of the most important tuning options for off-road buggies. Please use the pull down menu to pick the oil weight.
Kyosho also offers a smaller 20cc bottle of 1,000,000wt diff fluid (KYOSIL1000000) not present in this listing.
NOTE: It is not recommended to mix oils of different brands, as the thickness of oils of the same Cps rating may vary from different manufacturers! Stick to one brand for all your oils, so that you can reproduce the setup when its time to rebuild your diffs.