IT'S ALL ABOUT THE AMBIENT TEMPERATURE!
Yes, I was shouting, but not in anger, just to get attention. If any short term issues occur with 5w30 rather than 5w20, it will be with '04-up engines with Variable Valve Timing (VVT) and likely those would be in cooler climates. Overall, getting the skinny on issue of viscosity vs VVT operation is elusive. We get some anecdotal material from owners that say A-OK for a 30 grade and a few bragging about 40 grade, but these people don't necessarily know what's happening inside the engine. On the other hand, we have cautions from Ford, including some direct from engineers... but they are a bit short of specifics. It seems clear, even factoring VVT, you could most likely get away with a 5W30 in a hot climate. Might even be more desirable for a hard worker in a really hot climate, but in a cooler climate where the oil may seldom or never reach it's rated viscosity due to being below 210F... who knows. The VVT devices are controlled by the oil. It stands to reason that viscosity would effect them. How much has yet to be answered. Me, I'm being a little cautious. My truck is well outta warranty but I don't particular want to tear into it (well, I DO because I'm a gearhead and I love working on engines, I just don't want to have to pay for it).