It won't make much of a difference and you will probably not notice anything different from the different oils. Like said before, SM was the 'series' (if you will) of the oil when your car was new. Using a 'series' newer will not hurt anything. You can go higher in series SN, SO, but it is suggested to not go lower. Really though, you are using a true synthetic oil so it is better than using some BS oil that is an SL series.
I wouldn't worry about it too much. Especially since only two quarts are going to be the older series of oil. They probably had some old oil that never sold and they are using the sale to off-load it. Take advantage and go get some more while they have it. Just try to get all of the SN series if you can.