I've been through the searches, and I ended up here, more confused.

I live in Chicago, IL (+100 in the summer, -30 in the winter). My book calls for 5W-20 Motor Oil, but I can't really seem to find it that much.

I want to convert over to synthetic, and am thinking of going with 0W-30, 5W-30, or 10W-30. What I'm mostly concerned about, except for protection, is gas mileage. Which of these is best (I'm leaning towards the 0W-30 grade)?

Any ideas?

Actually, I think I found my answer here (http://www.sts.sae.org/servicetech/jan-2002/ford-sae.htm).

Are they right? 5W-30 in the 5.0L for best GM?

5W 20 is available in only a few brands, Motocraft and Pennzoil is all I see here in CT. 5W20 is used to meet fuel economy.
A 0W30 would be a suitable substitute and provide at least the protectin that the 5W20 would.

Kurt

From what I remember doing my research on the topic, there is not much difference between using 0W-30 and 5W-30 synthetic. IIRC, I was told Mobil 1 0W-30 protects to -52 degrees Fahrenheit, and 5W-30 protects to only -48. I arbitrarily chose 5W-30 for both of my vehicles because in this part of the country, it does not approach either temp. When it does, I am buying a one-way ticket to Arizona.

0W-30 does flow better at startup. i use it in my explorer, but i dont really notice any difference over 5W-30. but i use it anyway. i did notice a difference in my mazda 626. with 5W-30 or 10W-30 the HLAs would tap for about 20 seconds when you cold start the engine. when switched to 0W-30, the tap went away. so even the slightest improvement (even if it isnt noticable in the explorer) is worth it to me. they both cost the same.

Thanks guys for the answers, I'm going with the 0W30!

Oh yea, Sams Club has a case (6 quarts) of Mobil 1 for $22 - not too bad as it comes out to $3.60/quart.