Specifically NOT RECOMMENDED by the manufacturer. Or at least by Volvo. Car manual clearly says DO NOT use fuel additives
I dont know about Volvo, but the Honda manual welcomes it, with the fuel. The only thing they said about additives was that adding additives to the engine oil wont do you any good, except paying more for oil.
Also, alot of gas stations (atleast in N America) now sell gas preloaded with STP fuel additives.