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)

Ted, yes the oil in the transmission and the overdrive does mix as there is that hole between the cases and I guess the rear bearing in the 3 speed box could pass oil also. I was just noting that the manual for the R10, R11 overdrive says the oil you should use is SAE 40. It goes on to say you can use the SAE 40 OR SAE 50 OR 80 weight gear oil but the way they say it, it sounds like Borg Warner's preference is in that order. I only mentioned this as general information. Also thinking that  the thinner oil may help those that have a problem in cold weather. But to tell the truth, I think most of the times people have overdrive engagement problems in cold weather is because the solenoid is full of oil because the seals leak, and the thinner oil may just leak more into the solenoid. 

The Classic Thunderbird Club International (CTCI) for early Birds, ‘55 - ‘57, recommends 90W Mineral GL-1 gear oil when servicing the OD transmissions.   The servicing procedure is outlined in the link:    https://www.ctci.org/gilsgarage/servicing-the-overdrive-transmission/     I have a ‘56 Thunderbird with the T85 HD overdrive transmission and recently purchased GL-1 mineral gear oil at NAPA, part number NHF 65201.  I plan to service the transmission as I sort out this car. 

I tried to clip and post just the last page of this but could not get it to work. But if you scroll down to the last page or so you will see a section about what oil to use. ( see link at end)
 While I have always used 85 or 90 w oil in manual transmissions I thought that was the correct oil to use in this also, and was using it in this one. I have had some shifting issues down shifting into second, hard to get into gear, so I thought the syncro was worn or damaged. After taking it all apart the only thing I found was the teeth on the syncro were rounded off very slightly. I replaced all syncro parts and second gear with new parts, filled with 90w and test drove. It may have been a little better, but maybe not, at least not good enough. So I did some looking, I found an article that talked about using lighter oil 40 or 50w oil in transmissions and that it made shifting easier. I really did not like the idea of using lighter weight oil in a manual transmission but after looking at the Borg Warner manual again, seeing that they recommend not only 50w but even 40w I decided to give it a try. I got some 50w and after a short drive it is a lot better and was getting better the longer I drove. With the roads messed up with salt and melting snow it may take a while for me to get out and really test it but I think it will be good. 
As we all know, we should use 90w in a manual trans, I was worried about going with lighter oil and I only post this to help others that may have problems like what I had. This MAY help with hard shifting or a problem I see many complaining about with these over drive units, not wanting to go into or out of overdrive in cold weather. 
https://www.oldwillysforum.com/forum/TechData/BWOverdriveManual.pdf