this is common on 850's that are non turbo, just have the flame trap changed and be sure that there are no leaking seals, the oil cap seal is the first place the oil will come out (least resistance), you can also change the seal on the oil cap i changed mine they get very brittle over time and cost like 2.50 from volvo. be sure the oil has been changed on a regular interval, ask for service records, also call any volvo dealership and talk to a service advisor, give him the VIN # of the car and they can give you a service history of the car adn tell you if there are any recalls. Keeping the flame trap system clean is important as it leads to more problems if the pressure gets out of control.

it may very well be just a bad seal on the oil cap, but have the flame trap system checked and cleaned to be on the safe side.

the vacume lines to the flame trap should be good with no deterioration or cracks they should also be checked.

Dan