Here is a succinct article describing why Obama is using ISIL and others are using ISIS. Basically Obama sees the groups as a greater threat than to just Iraq and Syria (as in "IL) and rather is using Levant to refer to the former French and British colonial areas. So this is where we see if the "Bully Pulpit" is stronger than the collective press. Already a slight majority of members of Congress, but certainly not all, on both sides of the aisle are starting to use ISIL. Also in the link above, you can see the chart above and see the dates of when members of Congress started changing their terminology.