Mainly blowing off oil filters in the first few minutes after a cold start. All of the modular engine family have had some problems with this. With the Duratec it seems that the 99 model year seemed to have the greatest problem. It didn't take real cold weather to do it either since it just doesn't get very cold here in Southern California. We usually don't even have frost on the windshield more than about a week out of the year. Almost always if the oil filter blows out, the engine suffers rod bearing damage. I've only seen two Duratecs with this problem, but I've also seen more 4.6 SOHC engines do this than I can remember.
The main thing is that everything on these engines were designed with the thinner oil in mind. It just doesn't have the clearences necessary for heavier oil. This is really critical when the engine is still cold. The 5/30 oil will provide a much higher volume of oil to the lubricated parts much quicker. Warm ip is when the greatest amount of wear happens.
So do what you want with your car, but after seeing some of the damage done from using 10/30 in modern Ford engines, I'll try to keep it out of mine.
