If it was working good in its previous incarnation, I'd drain the oil, and check it for brass or steel bits. If it looks good, you can put it back in. It's a part-synthetic blend that lasts a long time.
If there is enough metal or other crap that you don't want to re-use it, you might want to pull the top and have a look inside. Then flush it with Varsol or something and refill with new lube. OD transmissions are sensitive as far as lube goes, I'd stick with the recommended stuff.