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I'm not familiar about the inline 6 in your car so I referred to a diagram at volvopartswebstore.com for guidance. Its most likely a coolant pipe as your car doesn't have EGR. It couldn't be an oil pipe as your car doesn't have any turbo plus turbo oil pipe usually not that big. Yes there are engines that use oil pipes to drain oil from upper crankcase back to oil pan but thats from different make.

Regarding smoke coming out of oil filler hole while engine is running it could just be the blowby gasses. Have you tried putting glove/paper test over the filler hole while engine is running - to check for any suction? There should be some weak suction there while engine is running (idling) BUT not to the extend that it sucks the entire glove/paper into the hole. Alternatively covering the hole with hand you should feel some suction there.

Regarding the oil leak at front of engine - despite having replaced PCV components - firstly find the source of the leak. Open the front plastic timing cover to see. Find whether the leak is coming from the top (camshaft seals) or bottom (crankshaft seal). Poor PCV system in white block engine mostly causes bottom leaks. White block engine has their camshafts intergrated WITHIN the upper cylinder block with high pressure oil channels for camshaft. This means possibility of oil leaks from within the upper block. If upper block is leaking you might have to dismantle the block to reseal the oil channels using anaerobic sealant specially made for this.

Amarin.