[v8volvo]

You may want to STOP for a moment and make sure this is right before you put it back together. Can you post a picture of the thermostat and gasket you bought, or post the part number from the box?

Remember this thermostat is an AUDI part and Volvo plays no role in manufacturing or engineering it. Even the tstats sold by the Volvo dealer are repackaged VW/Audi parts.
You probably received a thermostat and gasket by mistake that is for a GASOLINE Volvo engine since those do use a grooved gasket that goes around the perimeter of the thermostat. In that case not only is the gasket wrong but the thermostat may be wrong also. If the thermostat is for the wrong temperature or doesn't function the right way, have the correct block-off dimensions, etc, it will cause serious problems.

The friend you spoke to is wrong, the thermostat gasket for the DIESEL engines never has changed and continues to use an o-ring that sits outboard of the thermostat. The GAS engines do now use the style you described. Your friend might not have been aware that the gas and diesel engines in these cars have no parts or designs in common.

Rather than trying to make something work with sealant you should ensure you have the correct parts. Can you give us more info on the thermostat you have?

Don't run the car with this thermostat in it until you have confirmed that it is correct.