A Couple Simple (Dumb) Questions on My '46 with 350/350

[Dan in Pasadena]

I should know the answers to these questions but I'm looking for some advice.

I want to change the rear end gear oil and don't know what to use for certain. My truck has a Jag rear end of unknown origin - there must be some kind of identification on it but I haven't found it yet - But I know Jag rear ends are essentially Dana 44's so can I just use any name brand 90 weight gear oil in it? What do you suppose the capacity is? If not, what would you recommend? If you recommend a synthetic, since I don't know the mileage or history on this rear end is there any concern (as I have heard on higher mileage engines) in switching from dino oil to synthetic...leaking? Any other bad outcomes to expect?

Engine oil: I plan on using Castrol GTX 10w30; or I suppose I could go to straight 30w, as I have no idea what's been used in it in the past. It isn't using oil, it isn't smoking and runs excellent as is. I don't want to go to synthetic on this engine.

Last: The tranny is a TH350 and I've never had an older auto before. WHat fluid do you recommend? Since I don't know its history should I pull the pan and ...do what?

Pardon my stupid questions but you guys that have started projects before on cars/trucks that have engines/trans/rear ends of unknown origins must have an opinion on these simple questions. Any advice appreciated.