Fresh rebuild of my 68 differential

[OregonNed]

Yesterday I picked up my truck a second time after a complete rear end rebuild that started about a month ago. The first time they installed Sierra ring and pinion gears with a new Eaton Posi carrier - 3:42 ratio. The differential whined and howled pretty severely, so I had to take it back in. They either tried another set of the same gears, or installed another new set, and still had the same issue. Finally after the 4th of July weekend they installed a new Yukon ring and pinion - much better. I only hear a very, very minor amount of sound around 50ish, I am sure attributed to break in. So far I have ran 2 moderate 20 mile drives with complete cool-downs of the differential - and now in a few hours overnight I will be taking it around 70 miles (all freeway) to a big truck show. Per the break in instructions I will be maintaining around 60ish MPG for the next phase of 100 miles.

My real question is what oil to use after the 500 mile change, and what the best might be. It was finished off with Valvoline synthetic 75W-90 and additive added - however, I have researched conventional 80W-90 with additive is recommended by Eaton (GL-5 rated). Does anyone have any good advice on which oil to use when I make that change? Thanks in advance!