15x8 vs. 15x10

I have just purchased some 35x12.50x15 BFG Mud-Terrains and I already have some 15x8 black rockkrawlers on my truck would I have to upgrade to 15x10's to run these tires? Would it make a lot of difference between the two or would it make the tire look wider. I have heard of some people running 15x8's with these but the manufacturer says 8.5-11 are required. Im confused.
Thanks for your help.

 

[Burns]

Right now im running my 36x12.50.15s on 15x8s

I used to run my 33x112.50x15s on 15x10s

You will be ok with the 15x8 but the tire will looked more rounded on the sides. With 15x10s the tire does not do that