Lots of people that have replaced their fan only used the top two screws when putting things back together. None, that I know of, have reported an issue with vibration / wobble / excessive noise.

 

Certainly you could. Some people on the forum have reported doing so but I don't recall if they ever reported back whether it caused future issues or not. When I replaced mine I reinstalled all four screws. It only takes a few extra minutes and it's not like I'll be in there replacing the fan that often. If you do only use two screws make sure they are nice and snug...you don't want one backing out and falling on your motherboard and shorting something out.

 

4 yrs ago, my roamio fan started making a loud noise. Instead of going through the hassle of replacing the fan, I did the following:
1 removed round label from fan.
2. drilled tiny hole where center of label was
3. added a drop of oil in hole using needle point oiler.
Original fan still humming quietly

 

Did it a couple of weeks ago, the two hold it tight. Only takes a couple of minutes to remove motherboard.

 

Thanks all!

The fan was very easy, as was the mobo (even the HDD swap out for another 8TB was easy since I had a backup of all my recordings on another (3rd) 8TB drive which I used as a source for the new one I got this weekend when one died.
Unfortunately last night (created a thread for next steps), on the last step, the Cable card connector pins on the motherboard got bent, so I am buying a used TiVo Roamio 5 Pro from a family member (also with lifetime service) and will copy the drive in that one to a new 8TB unit to get the OS set up etc.
Then I will copy over my recordings using MRV (or use MRS to stream as another option someone provided).

Ironically I was just being proactive by replacing a working fan since it was 7 years old and I had to work on the unit anyway to replace the hard drive.
As someone replied (very nicely by the way) when I said that it was ironic that this happened because I was being proactive, "if it ain't broke don't fix it".