Use the 80W90. I have a 254 and use the same Coastal stuff and it quited it down quite a bit. My tractor is pushing 500 hours and zero tranny or front axle problems. I changed it at 50 hours then again at 350 hours.
I use the same 80W90 in my gear boxes on my post hole digger, finish mower, bush hog, ect. I buy it in a 5 gallon pail and got a hand pump for it from TSC the fits on top of the bucket and has a dispensing hose.
Chris