right, Mobil 1 is only thin in the cold, which means it flows faster than a 15W-40, meaning that the oil gets to where it needs to be from the moment you turn the key, unlike a 15W-40, so you get BETTER cold/wear protection from cold with the 0W-40
Once your up to temp its practically identical in viscosity to most other 5W-40/10W-40/15W-40 oils on the market today
So the only chance it has of seeping down valve seals is on a cold start, and thats only if your engine is worn
Mobil 1 do another formula which is 15W-50 Motorsport, this isnt much good for cold starts in the winter in my experience
My 1.4RT is currently on 98k miles, and im using a fully synthetic 5W-40, which will obviously be quite similar in viscosity to Mobil 1............and my car doesnt smoke.........in fact in 6000 miles ive only used about 500ml of oil MAX!..........Cold starts are a LOT better and the engine seems to rev better