I've read in the Volvo literature that the system has 3oz of oil in the evap, and 2oz or so in other components. Also the oil does leak out a bit when in cold weather the 0-rings shrink and lets out some gas...probably what you're dealing with now unless you have a leaking evaporator. The extra oil your adding with the gas isn't going to hurt. I just wouldn't go adding any more (like from just the oil can additive). The "goop" I was talking about was the stop-leak and some of the cans have other additives as well. These do nothing more than gunk up your system in the long run and should be avoided.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.