Aren't they both just 5-weight oils, with the difference being that the 5W30 will continue to flow like a 5-weight up to a higher temperature before it begins to shear, i.e., where single weight specs would call for a 30-weight (as opposed to a 20-weight)? It's my understanding that for multigrade oil, the 5W represents the cold flow rating, and the 20 or 30 represents the temperature limit at which the oil will still continue to flow like a 5-weight before it begins to shear, as measured against the rating SAE would call for in a single weight. Thus, the 5W30 would theoretically be the same as a 5W20 (a 5-weight), but work just as well over a wider temperature range -- up to the temperature where SAE would recommend a 30 weight single grade. And it's my further understanding that the reason such wider multigrade ranges are disfavored is that in order to achieve the wider range, the oil has to contain less base stock and more polymers, which break down faster. Therefore, although a 5W30 would appear to be a better choice, since it will work over a wider range of conditions, that is offset by the fact that the oil will actually break down faster. If true, that would tend to suggest that there is no practical difference in regular use, but Ford is more concerned that car buyers are going to exceed oil change intervals than the possibility that their cars are going to reach temperature extremes needing higher grade oils. I assume anybody visiting this website maintains regular oil change intervals, negating that issue, so the real question is whether the 5W- is as safe as the 10W- Ford used to specify, and not the 20 vs. 30 question.