While rebuilding my T85 transmission /overdrive I looked through the Borg Warner overdrive manual I have. I noticed they call for SAE40 or 50 weight oil for the overdrive. Knowing the overdrive oil is shared with the transmission, I was wondering if using this oil instead of the normal 90 weight would have any effect on the transmission, like making it easier to shift. Also, I have noticed a lot of guys have a problem with the overdrive engagement in cold weather. While I have don't have this problem I wonder if the lighter oil would help those that do. 
Just some thoughts that may help, anyone use SAE4o or 50 weight oil? 

I use to rebuild British Laycock DeNormanvil overdrives and transmissions. They also spec motor oil. I used motor oil and added 5% ATF.

I know some of the '80's Ford truck transmissions called for ATF, I figure it was for fuel milage gains. but not sure

Like Richard, I also run a Laycock De Normanville OD and the guru that rebuilt it recommended I use Redline MT-90.  I've only been running this transmission since March but it's been pretty good.  

The fluid recommended for Borg Warner transmissions is what is currently rated GL-1.  The current GL-3 to GL-5 oils are too slick for the synchonizers to work as designed.  The brass rings do not get enough traction on the spinning gears to permit a smooth shift from gear to gear. 

John - "The Hoosier Hurricane"

John, I have been using GL-1 oil from Tractor supply, they sell it for older tractor transmissions. The new blocker rings (brass rings) I have for this project have grooves cut in them that the old ones do not, I'm thinking these grooves are there to give the oil a way to get out when they get pushed onto the cone of the gear. 

Looking at the 1957 Ford Shop Manual, SAE 80 oil is recommended for both the conventional and overdrive transmissions.  Found no notes in the shop manual about the overdrive section of the transmission requiring a different oil.  The T-85 in my ’55 Ford (originally out of a 1963 Galaxy with a 352) has an inter-connecting hole between the main gear box and the overdrive allowing the oil level to be the same in both ends.  I have had no issues with the overdrive engaging as it should with conventional weight transmission oil.

Lorena, Texas (South of Waco)