Is there a solid carbide spiral router bit, down cut, that will cut a flat bottom dado in plywood, or any other material for that matter? If so, is this a good choice? If not, what IS the best bit for cutting flat bottom dados with minimal or no tear out?

Also, is there a significant difference in the numbers that are used to grade carbide, like 4 and 5, etc.? What does that mean?

Finally, I've seen ads for the 4 flute (cutter) Freud carbide profile bits? For folks who use the, are they worth the additional expense? Do they perform as advertised?

Thanks for your advice.