Curious newbie here, trying to understand if the different percentages of VIIs in 5w20, 5w30 and 10w30 oils affect the amount of wear protection and cleanliness an oil provides. Here's what I've gathered so far after LOTS of reading - please correct me where I'm wrong! 1) The more VIIs in an oil, the less actual oil in the formula. The lower the percentage of VIIs in an oil, the more actual oil is present. 2) Oils with a narrower spread between the cold start number and the hot number contain lower amounts of VIIs, while oils with a larger spread between the numbers contain more VIIs. So 5w20 and 10w30 oils contain less VIIs than 5w30. 3) VIIs thicken the oil but don't lubricate. It's the oil that provides the actual lubrication, so theoretically at least, the more oil the better. 4) VIIs eventually shear, causing 5w30 oils to thin down to a 20 weight. The shearing can leave residue in the engine. 5w20 and 10w30 oils are more shear-stable. 5) Thinner oils flow faster than thicker oils, providing better cooling. Thicker oils provide better film strength but take longer to warm up and don't flow as quickly at operating temp, which might cause higher temperatures? Given all of the above (or not, if I'm in error) here's my sitch and my questions: The Crown Vic's 4.6L modular engine was originally spec'd for 10w30, soon changed to 5w30 for better cold climate starting, but this got changed to 5w20 for CAFE standards. I've read that in other countries it's still spec'd for 5w30, even 10w30 in Saudi Arabia. I live in a desert where summer highs are above 110*F and winter lows average about 40*F. We occasionally have a few hours of light frost but I'm never out driving at that time of night. I put on about 9500 miles a year: about half a dozen short trips a week interspersed with two easy, 60-mile round trips that are mostly highway. No rush hours or stop-and-go traffic. The car has 119K miles on it. Before I bought it a few months ago it was a federal fleet car in NM, maintained on Motorcraft 5w20. It's current fill is Napa 5w20 synthetic blend and it's running very well, but I'm wondering if 5w30 or 10w30 would be better in this climate? If an Xw30 oil would be better, should I go with 5w30 or 10w30? I also want to make sure I burn off any water from the short trips. How much longer does a Xw30 oil take to get to full operating temp compared to a 5w20 oil? Thanks for your time. This is a great community. I'm learning so much!