I don't think that works, except that the higher viscosity oil should result is lower consumption. So if with 5w30 you were using 1/2 quart in 3000 miles and, for example lets say, the 5w40 used no oil in 3000 miles, then you could put 1/2 less in to start with and end up in the same spot. But in the real world, both these grades will need some topping up.
Is it that you don't want to have the remaining 1/2 quart laying around for several months?
Is it that you don't want to have the remaining 1/2 quart laying around for several months?