I don't know if this helps, but I know that on motorbike engines I have worked on that have been sitting, what happens is depending on how the engine was stopped, you wind up with one valve (or more) that is left fully open, on that valve that is open, the stem is then exposed to the elements via the intake or exhaust, thus you get a build up on that valve stem depending on how long it was left like this, the build up can be fuel varnish or even mild corosion/rust.
With something like this putting something in the oil is not going to help, but depending on bad it is, something added to the fuel may help remove some of the build up and stop the valve from sticking.

I don't know if the case is the same with your volvo, but I just though I'd add my 2cents worth...

:-)

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My 85 760Ti, and her name is Veronica...