Tractors use various fluids. The usual favorite is calcium chloride, if your tubes leak any or you get a puncture you need to wash that stuff off ASAP or anything it gets on will be rusty within the day. Other fluids are used too that aren't as dense but are easier to work with, beet juice, windshield washer fluid, water. When you fill a tractor tire you fill it so the liquid is up over the top of the rim to reduce sloshing. You want the liquid to stay in one spot and your tire move around it so you have less stress on the system.
A tractor tire runs over uneven surfaces at very low RPM. I have no idea what liquid would do in a car tire spinning at much higher RPM's.