I used 10-40 as I like my oil thick. The only thing 5-30 will give you is easier starting and better fuel efficency as the car warms up. My car always started even at -30, although a few times the first couple of turns were pretty slow. I especially would not recomend 5-30 if you do a lot of highway miles, as the engine runs plenty hot and needs the proper viscosity. That being said, if I knew that i was going to be just doing 5-10 min hops all winter I would use 5-30. I also use a piece a cardboard in front of the rad for short in town driving, and when it's really cold, I even use it to sheild 1/2 the rad at highway speeds. Volvo used to have an optional adjustable set up that could control airflow through the rad part # 656171-6 (but it looks like it was only on P120 ch-166399. P130 ch-84599. P220 ch-17949 so maybe it doesn't even fit later 120s) if you could control it from the cockpit that would be a great idea.
BTW congradulations on taking a 120 through the winter, I loved mine in the snow and it was the warmest car I have ever had in the cold. If you find studded tires you will have trouble getting stuck.