Typically you will be good with a 85-90 wt gear oil. Obviously it will flow better warm but unless something real nasty comes out just letting it drain a little while before refilling and you should be fine.
You are supposed to use a GL-3 gear oil, which isn't usually available except at farm /fleet stores. GL-5, the most common gear oil nowadays, can attack the copper in the trans (synchro blocker rings). There are exceptions, see below
I have flushed out my trany transfer box and both axles with 90 weight ..and after a good run and warmed up changed it all out for amsoil synthetic a lot quieter and seems to run cooler ..at least no gear oil smells now but I agree pull th eplug and let it drain for the night ..don't heat it with a torch ..I set fire to mine doing that ..yah I'm an idiot ..