[burly]

While Royal Purple may be superior to Mobil 1 synthetic oil, the difference between Royal Purple and Mobil 1 is much less than Mobil 1 and petroleum based products. If he is just trying to avoid buying expensive petroleum based oil from the dealer for his wifes car which probably doesn't get flogged that often, then the additional cost of Royal Purple probably isn't justified. It costs considerably more than Mobil 1, from what I've seen, as 5 quarts of Mobil 1 is ~$20 versus ~$35 for Royal Purple. No doubt important for very high performance engines, but for most vehicles, Mobil 1 will provide more than enough protection when changed at regular intervales. Heck, good petroleum oils will be more than enough when changed at regular intervals.

I'd recommend either 5W-20 or if you can't find it, 5W-30 synthetic oil and change it about every 5k miles if it is being driven under "normal" conditions. Or, if you want to go the petroleum based route, use the same weights as the synthetic, just change it every 3-4k.