I have been running 50 wt. full synthetic in my fuller transmissions for about 15 years. Some of my transmissions have been over 1,000,000 miles when I sold the trucks. Back in the 60's , 70's and 80's we put bearings in transmissions every 200,000 when we ran mineral oil. I have not been in a transmission since we went synthetic. Same goes for rears. We use 80w140 synthetic and haven't even had to go through a power divider. Oil additives may have had their place in the mineral oil day, but in my opinion they are not needed with synthetic oil and you could wind up putting in something that doesn't mix. Our engines are set at 1450 ft lbs of torque. Up to 2000 was rather common until fuel got so high. Transmissions and rears last over a million miles. What more would a person want.
