Just my two pence worth - I replaced my thermostat recently and did the bolts up without too much torque, as described in Robert DIY youtube video, and it unfortunately leaked. I took it apart and looked at the mating surfaces. They seemed to have a baked on pale residue on them, so I cleaned both sides with a little glass paper and tightened it together more enthusiastically, which seemed to cure it.
I'm beginning to wonder if the gunk on there was some old gasket seal and whether that is what Volvo garages where told to do, as it had only Volvo stamps in the book when I bought it.
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