That actually sounds like a good theory. And I'm pretty sure it's fuel smoke coming out from the exhaustion.You have either parasitic air (vacuum leak at idle) which the ecm is trying to correct by adding fuel, or excess fuel from another source ( whether that's oil or petrol you will need to determine)
Graph both O2 sensors (up and down stream), and ltft and stft at idle and 2500 rpms for at least 30 seconds and report back.
But why does it only happen when driving for 60km or more? If there's a leak somewhere shouldn't it be doing it all the time?
I'm thinking about doing a compression test to see if there's anything there that can guide me further.