Your dealer is probably correct as early S80s did/do have trans issues, especially '99s.
I've replaced several of them myself at our dealer and I can tell you that there's nothing easy about that job, book time runs about 12 hours labor.
There are no real SW issues when replacing one BUT considering the amount of labor to do that job, if you opt for used and it fails soon after its short parts warranty period, you might be out yet another trans with all related charges (at least labor again).
Volvo (GM) did at least fix their problems with their OEM remans as I have yet to see a reman fail and those come with a 1 yr, unlimited mileage parts warranty. A used trans might only have a 30 day or less parts warranty and if it's just another original '99-'00 trans, chances are good that it will also fail.
There is no thicker fluid, chances are that your fluid now still looks good (redish). The failures are due to internal o-rings failing and Volvo doesn't want them overhauled at the dealers, no training either.