I don't know about changing weights so I can only suggest you stick with 75W90. A lot of guys swear by Motul 300 Gear Oil. It's a full synthetic. Another popular one is Redline MT 90. You'll need 2.5 quarts to fill your transmission. I plan to run the Motul myself eventually. However if you don't do any racing, I'd just be sure to change the oil every 24months/24K miles and you can use just plain old gear oil, or you can spend a little more and put synthetic gear oil in for a little added peace of mind.
I just put synthetic in my car and have been running it in my wife's grand am for awhile now. Ran it in my XE also. Discussed this many times, so here's my take on the whole thing. Synthetic oil takes higher sustained temperatures to break down, therefore it has better potential to protect your engine. I buy the 5 quart jugs of Mobil 1 at Wal-Mart for about $17 a jug, that works out to be $3.40 a quart, and I've yet to see any synthetic oil that cheap per quart anywhere else. I've done the math and it costs me $60 a year, or $5 a month more to run synthetic over Dino, and I'm comparing it to $.99 dino, so if you figure, a lot of people spend more than that per quart for dino, then I'm spending even less than the $60/$5. So that's how and why I justify running synthetic. Nice thing is, you can switch back and forth whenever you want, you can even dump a quart of dino into your car if it needs oil and you've got synthetic in there, or vice versa, dump a quart of synthetic in when you've got dino in there. Why you'd do that I don't know, but if you were in a pinch and that's all you had, there'd be no reason not to do it. I don't notice any difference, but like I said, I use it for the added potential protection. There's lots of good info at Mobil's website about the benefits of synthetic oil, as well as some of the myths of synthetic oil.
OK not to sound like a complete idiot but where is the drain plug for the tranny? And where is it i pour tranny fluid back in? I'm used to my Auto Sentra i had before with a tranny fluid dip stick that i don't know much about my SE-R tranny fluid. Please Help me out guys i really need to change my tranny fluid but should i do that before or after i change my clutch ( which is coming soon)???