I plan to soon ('98 S70 with 160,000 km). I looked for it when changing oil and coolant a month or so ago, but couldn't find the check plug. Also I only had front ramps and car was not level so I couldn't do the check even if I'd found the plug. It seems it must be annoying to reach because the drain and the fill plugs certainly were not in an easily-seen and easily-accessible place at least under ramps (maybe they are further back than I thought).

I'd buy the MTF from Volvo dealer... that way you can't make it worse! It's probably expensive but you don't need much and the cost over 5 years or so will be minimal.