In working with the protein-oligo kit, I am curious about the use and amount of DMF suggested.

The DMF is required so that the S-4FB is soluble in the reaction mixture. The reaction requires a minimal amount of DMF (~33%) to keep the linker soluble, but above that amount, there is no affect on the reaction. It is important to dilute the reaction with buffer or water prior to desalting on the VivaSpin column.