You're in a cold, winter environment now? If you think your t-stat hasn't been replaced in the last 70K miles or if either the temp guage reads lower than normal and/or heat output is low, by all means spend $20 and a few minutes to replace the t-stat.
Otherwise, don't worry about about the stuff on the dipstick or underside of the oil fill cap especially if the car is often used for short trips, never getting hot enough to burn that stuff off. If the t-stat's OK, come summer you'll never notice that stuff doing nothing at all.
I see this every year on most FWD Volvos (and other cars) and it's normal. Unless you're seeing drain oil looking like a milkshake (VERY rare with Volvos unless severely overheated), don't let anyone tell you that you need to start a nightmare process of taking the engine apart as you'll be sorry, no doubt.