If you want your T-90 to last use it. these camera much prefer to be in constant use If your not using it. Dry fire it for a few seconds on high speed using a bunch of shutter speeds.
Yes the T90 does have some issue that show up so does every other camera ever made go talk to the Leica guys OH BOY>
I own two run the shit out of them and have never had so much as a burp.
Not every gobber should work on a T90 they are the single most advanced manual focus 35mm camera ever made. SO try to find someone who has experiance so you don't end up with a paper weight.
I personally would worry more about BEING alive in 20 years before I worried about a camera crapping out you can always buy another one if it does. It;s only a camera.