"Maintenance" and "thermal paste" as search terms brought up 0 results in the manual.  I also searched by "processor," and that yielded 130 results.  You're welcome to review manual.  PowerEdge T320 Manual  You'll also see other related pdf downloads.

Google yielded different answers for the lifespan of thermal paste.  Ranged from 2-3 years for gamers; some said 5-6 years and some 9.  Also depends on heat and humidity.  Please see post below.

Arctic silver paste comes highly recommended.  Link in post below is for a new one that's even better (Arctic MX-4).

If you replace the thermal paste, then you know when it was done.  Tip:  If the computer was bought used, I'd replace the thermal paste.  If bought as a refurb, thermal paste should've been replaced.  However the computer was bought, mfr. date should be on it.

Not always easy finding exact answers, but there's room for others to chime in also as experiences can vary.

Arctic thermal paste on Google

Arctic High Performance Thermal Paste Amazon - Described as lasting at least 8 years.