3-speed manual leaking fluid

The factory started to fill manual transmissions with DEXRON ATF back in the 70's and sometimes people would complain about the gears rattling so the fix was to dump out the ATF and fill with 90w. Either type of fluid will provide sufficiant lubrication. I like 90w in my manual transmissions, especially if they've got some miles on them. I live near buffalo and we get some cold temps (-15*f) here sometimes and having 90w in a trans has never given me a problem, yes the first couple of shifts when its that cold feel like stirring a cold pot of molasses but after a few minutes of driving it shifts normal. Also 90w is less likely to leak out of the seals.
Dont put any fancy synthetic fliud in your trans, some of that stuff is too slippery for the syncronizers to work properly.