I purached a F.M.S. 3.73 punkin for my blown bird. I am going to replace the fluid that came in the carier with a senthetic gear oil. I don't know which one to use mobil fully senthetic 75w-90 or vovoline 85w-140 senthetic blend. I am leaning toward the mobil. How much fricion mofifier should I use in the rearend? I need to know this by 9:00 am cst wens morning That is when I will finish the install.