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I have a 2003 6 liter with 90,000 mi . When its been sitting overnite, it starts with a rough idle for 5 seconds then smooths out and is fine the rest of the day.
I'm from Texas but working in Colorado right now. I've experienced the same problem and thanks to posts I've read in here I switched from the Motorcraft 15W-40 standard oil to Mobil 1 Synthetic 5W-40 Turbo Diesel Formula. It smoothed right out. and idles better and just overall runs better with the sythetic. Hope this helps.....
The Mobil 1 will run you at around $7 a quart and you can buy Royal Purple for around $8 a quart at your local parts store. I've been buying mine at Pep Boys. They carry both. So for a filter and oil plus tax will run you around $125 to $130. Not sure where I'll buy it once I get back to Central Texas Guess I'll turn to the internet....