I worked in a motocross suspension shop for years so this is my take.
Suspension Servicing Schedules are wildly variable. How many hours have been ridden? What kind of conditions? How fast or big is the person riding? etc etc. And just like any mechanical thing, Cheap oil and grease that is clean is better than expensive oil/grease that is dirty from 100 hours of use.
Anecdotal information; When I worked in the shop, I played with pure nitrogen vs pumping up suspension bladders with "regular" air on my personal bikes (a fun luxury of being "in the biz", I could fiddle with things A LOT more than the average person, for free!) I couldn't tell a difference on the shock dynos or in regular riding. Maybe a pro could? Maybe with an MTB being so light, and completely dependent on the air spring it would be noticeable.
"Regular" air is about 78% nitrogen so its theoretically 22% less heat stabile than nitrogen I guess? I would assume the difference would be most noticeable once the suspension starts to get *HOT* and the normal air is expanding more than nitrogen would. (Dirtbike suspension, especially the shock, gets REALLY HOT compared to *most* mtb stuff.
I never tested air vs nitrogen for long term for seal life because suspension service life is wildly variable and the outside world we use our bicycles and motorcycles isn't a lab environment.