Sounds like you've got a problem with excessive internal engine pressure. A few things could be the cause. Has the flame trap and/or oil trap been cleaned recently? If not do it ASAP, clogged traps can cause this pressure buildup. The other thing that comes readily to mind is a blowby problem. This is a case where your piston rings have worn to the point that when the engine goes into the compression stroke, the pressure that is normally built up inside the combustion chamber is leaking out past the bad rings and escaping into the crankcase, causing higher pressure there. The fix for this is nothing short of a complete engine rebuild or a swap. This can be diagnosed with an engine leakdown test, which any competent mechanic can perform. Bottom line though, I seriously doubt you've lost 1.5 quarts from the dipstick tube and filler cap alone. Check under the car for puddles, the high pressure may have caused an engine seal to blow. However, if you fix the seal (an expensive proposition in itself) and don't address one of the causes mentioned above it will simply happen again. My advice, find a good indie Volvo specialist ASAP. In the meantime, if your driving a long distance, keep lots of spare oil in the trunk and check the level often, like every day or every gas stop, whichever comes first.