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?