The throttle body is easy to clean - can even be done by home types with two left thumbs - as long as you have the FAQ or manual and a 13mm wrench. When you talk about oil leaks and O-rings, I am not sure what you mean. A careful inspection of the engine with the car on a lift will usually determine the source of an oil leak, and if it is sue to a flame trap problem, the often, cleaning of the crankcase breather system will eliminate the leak unless a seal has actually been blown out of the housing - then you will have a MAJOR leak. The only O-rings that are of concern are the ones between the oil filter housing and the block and the filer housing and the cooler connections. This is awkward to get to on the turbos, but a reasonably simple repair and the problem is obvious by inspection on a lift.

It is not common for there to be a rear main seal problem on these later Volvos - particularly with a clogged breather system. The leaks will usually show up at the front of the engine in the crank, cam, and idler shaft seals, and often the cam or idler shaft seal can be pushed out of the housing.

Be sure to check the oil sender for leaks - another common problem. Right side of block, toward the front, below #1 exhaust port.