Wow, this topic has really taken off. I thought I'd give it a day for a response. Once again I'm probably on the right website.
Here goes: My car has 22K miles on it when I switched to Castrol 0w-30. I've owned quite a number of performance cars including 3 Mustang GT's and when I'm telling anyone that the car is slower I'm not pulling anyones leg. As for cold weather start-up's; the car has responded better with the Ford 5w-20. My car also had at least another 5 hp with the Motorcraft oil, maybe thats were the filter comes in. On my Vette, I immediately noticed a difference when I switched from a dirty Delco air filter to a K&N. I know what your thinking, but, would you throw away a new/good AC Delco filter for a K&N.
On the Stang: My idle is fine, and the car runs smoother with the Castrol. My car doesn't have alot of milage.
When I first bought the car new: I changed the oil. Then again at 5000km, then at every 5000km intervals. So around 8 times with the same Motorcraft 5w-20 oil, Motorcraft oil filter, and a K&N air filter after the first 5000km.
My 1 month review is based on the Castrol 0w-30, K&N oil filter and the stock OEM Ford air filter. After 1 month of driving and 5 tanks full of gas, my milage went from 400kms (250 miles) per tank to 275kms or 172 miles per tank. This is approximate, and for all 5 fill-ups.
If the Ford oil isn't hurting my car, does anyone recommend going back to a 20wt oil?