It is very uncommon for the rings to fail in Volvo engines. Adding oil and having the compression go up is an indication that the valves are not the problem. Oil can also seal a head gasket leak as well as worn rings.

Use a magic marker and note the coolant level when your engine is cold and hot. Then check and note the level of oil on the dipstick with a hot engine after turning off for a minute or two. Check at least once a week.

Oil can only cause white exhaust smoke if it gets into your exhaust in an unburned state, otherwise it will be blueish. Black is fuel. White is normally antifreeze/water which also means condensed water in the muffler.

Driving the engine 'hard' for 10-15 minutes lets the valves rotate and keeps them clean, which is a good thing. 'Hard' is above 2500 rpm. 5000 rpm is used to clean out carbon deposits.

AutoRx can be used to remove gunk and baked on oil deposits inside the crankcase and will help keep the oil separator clean, which then helps keep the PCV clean.

Klaus
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Proud owner of a 220S. If I had more room, I would have more Volvos.