Tamiya, Bragdon,Ak, Doc`s are all good weathering materials. But if you are looking for the mud look you can`t beat the real stuff thinned with water and flicked off a toothbrush. Quick tip: if you do not want it to come off with handling add a little white glue to the mix.
Check these out;
http://z8.invisionfree.com/Scale_Racing_Lobby/index.php?showtopic=39
http://z8.invisionfree.com/Scale_Racing_Lobby/index.php?showtopic=38
I sift my dirtand then put it in Blender. You can make it as fine as talcum powder if you want. It can be used as is for dusting engines, tires and interiors.