I cant see the harm in increasing to 10w-40, especially in a hotter climate like SoCal. However, I think I would first try the 5w30 with one quart of it replaced with a quart of Lucas Heavy Duty Oil Stabilizer or maybe better yet a high mileage 5w30 - Valvoline Max Life High Mileage Synthetic Blend is a good choice, as is the full synthetic - with one quart replaced with the Lucas High Mileage Oil Stabilizer. I did this in my van (listed in my sig) and it hasnt lost any oil so far like it normally would. Normally it would lose a small amount - probably around 1/8 a quart or so - in 1k miles. Hasnt lost any yet.
I dont know how many quarts that crown vic uses, but the recommended ratio for the LHMOS is 20% of the Lucas to 80% of the oil. In my van, that just happens to work out as 1 quart with the 5 quart capacity.