The brief
The Touran had visible oil leaks, a burning oil smell, decreased engine performance, increased oil consumption, and engine smoke. He brought it in. On a turbo engine the turbocharger is fed and drained by oil hoses, and on this one those hoses run hot and over time the rubber perishes and the joints loosen, so engine oil weeps out, which is the leaks, the dropping level and the burning smell when it drips onto something hot. A turbo that isn't getting a clean oil supply and return doesn't perform as it should and risks damage, which is the lower performance and the smoke. Perished turbo hoses only get worse, so they need replacing.
The diagnosis
On the lift the leak traced to the turbo oil hoses, the feed and return lines, perished and weeping at the joints, with the turbo itself and the rest of the engine seals checking out. So it was the hoses at fault. That's a hose replacement, with fresh seals at the joints, rather than leaving perished rubber to keep weeping and starve the turbo.
The work
The old turbo oil feed and return hoses were removed, the joints cleaned up, and new genuine VW-spec hoses fitted with fresh seals and clamps so the whole path was sealed. The oil that had tracked down was cleaned off, the engine topped to the correct level, and the system checked for leaks. A road test confirmed it stayed dry, the performance was back, and the smoke was gone.
The outcome
Dry turbo hoses, no oil leaks, the level holding, no burning smell, no smoke, and the performance back. The Touran went home with the turbo oil supply sealed up. Perished turbo hoses only get worse, and a turbo run on a leaking supply risks damage, so changing them when it did kept it to a hose job rather than a turbo one.