Old wives tale or not, I've always had bad results running something other than synchromesh in a full synchro transmission. I didn't know any better and ran a gl5 in 3 of my geo metro 5 speeds. Took about 2 years each before they were trash. Old GM mechanic told me to run sychromesh since I didn't know what really was supposed to go in there and my current 2 are still nice and healthy after 5 years. The saginaw in my truck I ran a thinner synthetic in for about a year, it seemed shift harder for some reason, ended up swapping back to TH400 before trying different oil. I've never used anything since but the Pennzoil syncromesh and had no problems at all even in the real cold weather.
Check out this article I found that talks a little about oils in sycro trans,
http://benmlee.com/4Runner/Transmission_Oil.htm . Not sure if the friction modifier thing applies to a saginaw or not since this article was mainly about toyota trans.
Bottom line from everything I've read, you need a good GL-4 if you can find it for a trans with yellow metals and synthetics seem to be a bit thin for the saginaws. Most of my saginaw problems are not related to oils, but are caused by my right foot I've been told.
Some people are like slinkies. They serve absolutely no useful purpose, but still put a smile on your face when pushed down a flight of stairs.