Hi, all. Just joined the forum. Apologize for the following long-ish post, but need to provide some background first...

So, I just bought a 2008 Sentra 2.0 S with the CVT transmission. There are 120,600 miles on it. And I am a bit worried about maintaining the health of the CVT, though there are no symptoms (yet) of transmission trouble.

I am the third owner of this vehicle. The second owner sold it to me after only driving it for about 500 miles, and if I had to guess, it would seem just more than a coincidence that the original owner sold it to him as soon as the extended CVT warranty ran out at 120K just a little while ago.

This second owner told me that the original owner told him that the CVT fluid was changed at the 100K mark, though the Carfax report doesn't specifically indicate that this was actually done. It does show unspecified maintenance at a now defunct mom-and-pop Jiffy Lube kind of place around that mile mark, which I learned used do that kind of work. However, of course, the job wasn't done at an authorized Nissan service center.

Because I don't know for certain if the fluid was changed 20K ago, I'm not sure whether to get the fluid changed right away or not. So I called two local authorized Nissan service centers:

Nissan service center 1: "You need to get the fluid changed every 30K miles AND we strongly recommend paying to also get the TCM reset/reprogrammed. Total cost = $250+tax".

Nissan service center 2: "The fluid is supposed to be changed every 60K miles. And there's no need to do anything with the TCM unless there's a problem evident. BUT there's the possibility that changing the fluid could cause a transmission problem at the mileage because the fluid acts as a detergent and could loosen up any old particles. However, we see this problem more on the 3-speed automatic after changing the fluid. Total cost is $200+tax."

Wonderful. What the heck do I do?

Beyond that, I've read that there are auxiliary transmission oil coolers to install that could help preserve the CVTs, but service center 1 seemed to be ignorant about them. Is there a particular one for the 2008 (or that generation's) Sentras, and how would I find a mechanic who would know how to install one properly?

Finally, the manual indicates that I should use "regular gas at least 87 octane". Would it harm the engine to use premium, or is that overkill? Use Plus (89 oct) instead?

mac , many people including me subscribe to using "the next higher than suggested" grade , in your case, yes , going up to 89 when ambient temps above 100 deg. F .Not going to go into detail , many will argue one way or the other, screw talk - for the sake of the car JFDI (just # do it)
In the CVT , @ 120kmi and "no issues yet" ?? That's a good baseline - BUT your lucky , and in my opinion in order to continue to have little or no CVT problems - gotta change that fluid WITH a pan drop , blow-out screen or REPLACE If 'n a "sponged filter"( they get "cloggage" ).My opinion is NOT to save $20 with aftermarket fluids - use the proper ( as specified 1-eng. cmpmt. sticker . same as in 2- original Owner manual ) NS designation. I do myself , have Dealer receipt (cred. card ) and take photo / video of key areas like "particulates" in pan , NS-x bottles used , color of fluid etc. I don't know or care too much about the TCM "register" of the procedure , cause of the aforementioned documentation I do.
The most important thing is doing this , and doin it RIGHT ,+ documentation , I know my DIY is as good or BETTER than ANY Dealer.But , of course you can find a Dealer you trust ? And who will do a FULL pan drop & Filter / Screen job ?? Nothing wrong with that , that's what "the other half" does.

Thanks for the response. For the moment, I'm not confident enough in my own abilities to the do the CVT fluid change myself, so I'll have to have a shop do it. I'll see what it costs for them to replace the filter, etc.