If there's ever a good time to use up the 10w-30, now would be that time. It's starting to get warm, so the 10w-30 won't put as much of a drag on the engine at startup than it would in the colder weather. Also, common knowledge is that 10w-30 is more stable that 5w-30 in hot weather (although 5w-30 has certainly come a long way). I agree with the others who say that it depends on your driving, though. If you're more prone towards longer trips, you'll probably see no difference. If short trips, you might see some drop in MPG compared to short trips. I can't ever see 10w-30 causing harm in an engine spec'd for 5w-30 except in very cold weather.