Here's the write-up I used:
http://vbxmaxima.8m.com/mtoil.html
As you might know, the service manual and Haynes instructions of this procedure (which are the same) are pretty lame. I had to get a HUGE breaker bar (like 2 feet long) to get the damn filler bolt loose (I tried y smaller breaker bar to no avail) and I used an open-ended 12 mm wrench for the drain bolt (can't get a close-ended wrench around the bolt since there's a bar right next to the bolt...). I got the long breaker bar at AutoZone for 17 bucks. I got the Amsoil gear oil at amsoil.com. Here's a link to the oil for our trannies:
http://www.amsoil.com/storefront/mtg.aspx
As far as changing the gear oil goes, for our cars, it's recommended (and necessary if you look at just how vulnerable our tranny is) that we change out the tranny gear oil every 60k miles. A few people on a site all about oil,
http://www.bobistheoilguy.com, argue that the gear oil should be changed out sooner, like every 30k miles. I got my car in April with 66k miles on it without knowing much about its history (I doubt they did the 60k service, after close inspection), and now at 73k, I have read about both manual and auto tranny failure in our cars, so I went researching how to change the gear oil and what oil to use. With my Civic, I just had my mechanic change it when he did my clutch for safe measure (and I saw his BG synthetic crank display...) and since that car had 160k miles on it at the time and probably was running the original gear oil...
. Just like you don't want to be running dirty worn out and thinned-out oil in your motor, both performancewise and wearwise, you won't want to have rotten gear oil in your tranny, leaving the internal vulnerable to all kinds of wear issues. Here's the maxima.org page all about gear oil stuff (links and all):
http://forums.maxima.org/showthread.php?t=225766
Notice that they stress getting GL-4 oil. If you go ahead and use GL-5 (much more readily available and used in most newer trannies), the EP additives in the GL-5 will eat away at your synchros and other parts made with similar metals, so be sure to get GL-4. I chose synthetic since I live in NY where it gets cold in the Fall and Winter, and the synthetic runs better both when cold and hot. When I switched in the synthetic amsoil, I noticed within minutes that my car didn't slow down as quickly when I let off the gas in gear as it did with the old gear oil. My educated guess is that this is attributable to lower resistance with the synthetic.
I've been trying to get the tranny fill bolt off for weeks, and I finally got it off this morning with my newly-acquired extra-long breaker bar. I got Amsoil after reading lots of praise for their motor and tranny oils on maxima.org and bobistheoilguy.com. Let me tell you, the rough shifts and notchy shifter were gone within 5 minutes of leaving my garage with the freshly-filled tranny. After stopping for McDonalds, I decided I had to get him into 5th, so I head up to Hyde Park on Rt. 9, make a right onto St. Andrews and gun it. No shakes, no roughness, no shudder into 4th or 5th. Buttery smooth shifts and what seems to be more direct power transmission. I get onto another desolate straightaway (mind you this is 4am) and, when I go to shift into 3rd, the shifter slings into 3rd as though it could have done it on its own and the gear was perfectly synchronized. Just utterly impressed and relieved that it feels so smooth and quiet. Now, I can proudly display my Amsoil license plate holder and drink from my Amsoil coffee mug. Huzzah 
). As far as I know, we don't have to worry about sludge build-up in trannies like we do with our engines, and I don't know of any tranny flush anyway. Before changing the gear oil, I ran the car for a few minutes and moved around in first and reverse to get the oil running, for safe measure 
). The tranny was barely warm and the gear oil was barely warm, but it still came out as quickly as my engine oil comes out when I change it hot. What's weird about manny trannies is that the only way to check the fluid level is by opening the filler bolt and it's full if you feel oil when you stick your finger in there