There are a lot of mixed reviews of Seafoam on this board. I like Seafom a lot, I use it regularly in the PCV line as well as in the gas tank. I've used it in the oil as well.
When I got my 99 Rodeo two years ago it made a ticking noise at highway speeds. At first I thought it was a sound in the dash area. I put seafoam in the oil and let it idle for ten minutes, took it for a drive (2-4 miles) followed by another short idle.
Ticking has never been back. I am not the most mechanically inclined person when it comes to the inards of an engine, so I never pinpointed it, but it's gone. I did another seafoaming next oil change. On the next one, I used Auto-RX. There have been a few OCI's since. Next week I'll be changing oil again, using Seafoam. Also through the PCV line and the EGR system.
A lot of people call it a harsh solvent, it is not harsh. It has been used for years.
I'd say use it, but not at the same time as ARX, they'd conflict eachother (I'm assuming they'd conflict, I don't know). Use the ARX if you have it and in the future use the seafoam. I like both of these products.
Obviously from your reading on the Volvo forum you see people like it. Many people like seafoam including me.
Marc