During a "live chat" on swedish newspaper website he mentioned they lost oil pressure on BOTH engines (one shortly after the other). Any clues why this could happen?
Sounds like ice and bad preparation.
When an aircraft, that is not designed to live in icy flight conditions, encounters ice the engine breather exit can ice up. Plugging this hole first causes the oil pressure to climb.......until the excess pressure makes itself another exit - blows a seal or even a filler cap. Oil leaves the engine in a hurry. This lack of oil then shows as reduced oil pressure.........if not stopped quickly the engine can then seize/fail.
Pilots who need to fly low performance aircraft in arctic conditions generally make a small modification to the system - lag the breather pipe and cut a slot in it within the confines of a nice, warm cowling. This gives the engine somewhere reliable to breathe and helps keep the oil where it works best.
One can only assume that these were not experienced ferry pilots. They are a lot more experiemced now!