gear oil help in my 79 4x4 f350

[gregamorton]

I added the diff covers and changed out the fluid to lucas 80 90 synthetic. I got me a few clean tubs to catch the fluid because I wanted to put tape on my threads so I could get rid of the very small drip every so often. The rear fluid looked awful and I thought I drained it out well so I threw it in my oil barrel. I will let drain till tomorrow night and add my last 3 quarts to fill it back up. The front was super clean but im thinking I want to go to a thicker fluid because I can hear just a bit of clatter and think its because the synthetic fluid and a bit wore spider gears. What fluid would yall go with? I was thinking standard Lucas and 85 140. opinions??