Thought I'd post here since this thread was already open.
I just did this job tonight. Don't know why, but I thought this job was going to be seriously aggravating. I am not kidding when I say it took me 20 minutes total, from beginning to end, and that include cleaning up the oil mess. Thoughts:
1. I went to Nissan and gave them the original part number. That part was superceded and they told me no longer available, meaning you have to get the "new" oil ring or do the "Fram" fix (do a search if you want more info). I took the new oil ring and figured I'd see what happened.
2. The tech manual tells you to unhook the hoses, plug them, etc. You *do not* need to do this. Once the cooler is off you can twist it fairly easily and see the back side. The tech manual talks about replacing the check valve, but all the threads I read on this no one bothered and no one reported problems.
3. You need a deep well 22 MM socket to take the bolt off. I got an impact one from Lowes for 6 bucks. It came off pretty simple. I used a breaker bar but I could have got it loose with just a ratchet.
4. The "New" O-ring (I have an 04) is black, the old one is orange. When you put them side by side, the new one looks like it's too big and won't fit. It will fit. Just put it in the back of the cooler and work it around, on first glance you will think it's too big and won't fit but once you go all the way around pushing it in easily with your thumb it slides right into the groove at the end.
Once I was done I started up the truck and let it run for a few minutes, and everything appears to be nice and tight. I'll check it in the morning, as my old gasket would drip all night, but everything was clean and I think the problem was solved. If so, this has to rank as literally one of the easiest repairs I've ever made on any car, much less the Titan.