Dont use gear oil, the noise its making is not because of using the "correct" oil. The bearing design and chain are the reasons for using thin oil. If I was going to change the oil at all for something a little thicker I would only try something like 30-40 weight motor oil but do that at your own risk.
How do you know its getting hot and what are you comparing the heat too? I would say that if you have noise and possibly excessive heat then you probably installed something wrong.
How old is your tera low case? When was the last time you have taken it apart? If in fact you have a tera low 4 to 1 Front case and depending on the age and abuse on it, your planetary is most likley getting ready to go out on you. Those cases are noisy new out of the box and they do require more maintenance than regular tcases. Try using an ATF+4 if its not to late but depending on whats happened the large ring gear and straight cut planetary gears are most likely the culprit of your noise.