[240 2NR]

That milage isn't very high at all. I have a 93 that's about to roll 70k miles and it runs fine on full synthetic 5W-30. I'd consider 150k to be the start of high mileage if well cared for.

Now my understanding is that the low number is it's cold performance and the high number is it's hot performance (viscosity). Going to a 5W-40 from 10W40 should only be a "lighter" weight oil when cold, but should act the same as your 10W-40 once up to temp. I don't think you will find a significant change in perforamance, all these weights are close. I believe tnord runs something like 15W-50 in his 240. My guess is the sluggishness is either from overfilling or another problem entirely. I don't think just a 40 weight oil would do it.