Mid 80s is correct for a working thermostat. The dashboard gauge is a load of nonsense and fudged to always read 90 from 80 to 100 degrees.
That and the clause about oil consumption in the handbook is purely to stop people pestering the dealers/VW.
The way to spot a dead thermostat is rising temps in traffic which drops off again sharply when rolling. A working stat will hold a constant temp for ages.....unless your fans are fooked.....and on that score, I recommend people periodically check they come on. You can do that by doing an output test in VCDS, putting the A/C on, or letting it idle at home until they come on. If the needle heads towards 120 degrees and they're not coming on.......you have a problem! Better to confirm it at home rather than in a line of traffic.
