I spoke with a guy today who ran engines for the Navy on air craft carriers. He knew a great deal about lubricants and of course synthetics. According to him if you are not using 5w (or 0W)-20w on newer engines, which spec that viscosity, you may be doing some damage at start-up; which is when the majority of wear happens in an engine.
Even though Mobil 1 is a superior oil, the right viscosity is necessary for these newer, tighter engines.
So, what visc Mobil 1 were you using!
Now even though a 5w-20 and 5w30 have the same low number, indicating they should provide the same lubrication when cold, this is apparently not the case. It actually takes longer for the 5w-30 to start lubing the top of your engine.
By using Mobil 1 5w-30w I thought I was providing better lubrication at the high temps as well, but the 20w is sufficient and at a lower viscosity lengthens the life of the engine, provides better mpg and lower emissions.
So here is the kicker. I switched to a 5w-20 organic oil today ( I can't locate the 0w-20 Mobil 1 in my area and neither Castrol, Penzoil or Valvoline make a sny in 5w-20-yet, though they are in development) and guess what?-I think my engine runs quieter! This may be all in my mind, but you know, I think it is quieter!
So, if you were using Mobil 1 5w-30, I think you are making the right decision going back to OEM specs.
BTW, if you were using the Mobil 1 oil filter, that could be a problem as well. Apparently it does not have an anti-drain back valve and the position of the filter on the Zetec definately requies one. This fact alone would cause more wear at start up than the wrong visc oil.
So, I'll probably run a syn in the future but will be sticking with a 5w-20 weight oil.