If there are oil all over top of the head when you change plugs, the PCV system is likely clogged and I think the oil is pushing out the PCV hose or oil filler cap on top. There might be puddles of oil in some spark plug coil holes and also towards the exhaust side of the head since its tilted. Here is how to service the PCV system (link).
Volvo techs on the websites seems to suggest that nearly every P2 car has a leak here. The only reason we don't see it on the garage floor is because the belly pan prevents it from hitting the floor and the leak level is low enough that it gets all dried and gunked up.
There are 2 leaks from the turbo. One is the typical oil drain line leak. The leak seems to be from where the line enters the block/oil pan at the bottom. Older cars have the orange o-ring that leaks (you can't see it until taking it out). The new o-ring is green colored. Replacements on the none AWD cars are much easier because the angle gear is not in the way. Directions for the none XCs are here. More on the XCs below.
The other leak is from turbo air intake. The plastic pipe seems to not seal tight enough and oil leaks out and migrate towards the bottom of the turbo housing and drip. Volvo mechanic on the forum seems to note they use bicycle inner tubes to make a seal. I've just tightened the hose clamp and the drip pattern shows it is quite a bit reduced. I left some notes at this link after taking the fresh air pipe (the one that provide the air intake) off to check the PTC valve.
Actually, my leaks seems to have stopped after I took the fresh air pipe off and cleaned the oily gunk that collected on the bottom of this pipe's fitting to the turbo.
Now, taking off the turbo oil drain line on the XCs are tougher. It hides above the angle gear and don't have a lot of space to maneuver tools in there. I managed to do mine but found several difficulties
During the process the pushing and pulling it out, the old o-ring got cut in half as I was trying to force the drain line out. Only 1/2 of the o-ring was recovered. I'm guessing the other 1/2 might have gone into the engine :( But it is rubber so I hope it just got chopped up and stuck inside the oil filter. Here are the directions if you really want to try it. You need like the perfect T30 torx bit (maybe one with a tip that is about 1.5 inches long or so) to reach the torx bolts.
Here are a couple of more links to discuss the difficulty of this turbo return link gasket job on the XCs.
The oil filler cap has a rubber gasket that hardens over time. This allows very slight oil leak around the filler neck. New gasket seals tightly and is cheap. Doing this with 30k service is probably good idea.
I read the hose connecting to the of the fluid reservior is a common failure point. But I appear to have a very tiny leak where the power steering pump's spinning shaft is. There isn't enough leak to get dripping PS fluid (See the Steering section for discussion on PS fluid type). Just a little that gets mixed in with the dirt in the engine to give it an oily dirty film around the PS pump. Onto replacing the PS pump with with rebuilt soon.
The leak is very slight and small traces can be seen at every oil change. Some oil + dirt residue also can be found on the bottom frame/member below the crank pulley. My independent Volvo garage suggest I watch this closely so it doesn't get worse and nurse it to 105k miles and change the seal with the timing belt (since the timing belt has to be removed to change the seal). This oil leak maybe related to using synthetic oil. I purchased the car at 45k miles and use synthetic oil since. This oil leak may have started around 55k miles. At 109k miles, it seems to slow to a trickle and probably isn't an issue anymore.
If your 01's Viscous Coupling has oil on it. Chances are the rear differential's pinion seal is leaking. Have to take the center shaft (see here), take the VC off to service this seal. I need to do mine as my VC has oil on the bottom of it. Here are the directions after the center shaft is out.