That is something to consider. I have 535 lb/ft of torque. The 855 is supposed to take 600+. I did use GL4 80w90 conventional in my 833. It is a junkyard trans that I never rebuilt and I took it down the strip at least 200 times without issue. I would think that 75w90 would be ok in the 855, but I would hate to destroy it.
