Gear Oil viscosity?

I'm about to fill up my new 3rd member and I'm wondering what gear oil is better? I usually use 80w/90 but the guy that set up my rear (full spool) said to use a good synthetic or Lucas. I like lucas so I picked up 85/140 wt oil instead of the 80/90. Anyone use the 140 or is this too thick. I figured it would last longer and would be better for more extreme torque applications.