You can definitely go back to a 5W-40 from 10W-40 - I did this myself and have experienced no problems. From what I've read, going from 10W-40 to, say, 0W-30 isn't advisable in a single change; it's better to gradually work down. I don't think 10W grade can be fully synthetic, whereas 5W usually is, so that would be another reason to change.