The forecast calls for the rain to start next Monday and continue for six months after that, so definitely expect lots mud. I'm still using the stock tires that came with my bike (Maxxis Mocust CX), but I've got Michelin Mud2's high on my wish list.
What Mongo is referring to, however, is the bio-mud in the rodeo arena at the fairgrounds. I find it's best not to think too much about what's actually in there.
Anyway, if it's like it was last year, the tread on your shoes may be more important than the tread on your tires. I saw a couple of juniors lose their shoes in that pit and finish the race barefooted. Those young guys are hard core.