Proper weight of gear lube oil in differential

[mrdonut12]

I've got a slight dilemma regarding which gear lube oil
to run in my differential.
I consulted an industry expert who owns a heavy duty
fleet maintenance shop and works on trucks all day. He
said that GL-5 80W90 gear lube oil would be fine in my
Rockwell 5.29 gear ratio differential.
That's what I put in.
Digging through my Blue Box, I accidently ran across
some papers that stated Rockwell advised using 85W140 oil.
Since my Bird is so old, I wonder if lubrication
technology has made that recommendation obsolete, or if
I should switch to that weight.
The lubricants made in 1981 are a far cry from what's
available today.
The book also stated that, if you drive less than
60,000 miles per year, the gear lube should be replaced twice
a year, Spring and Fall.
That's what I'll do. Any experience on gear lube weights?

George Witt
81 FC 35
Lincoln, Nebraska (60 miles down the road from the
College World Series--Huskers play Friday night)