Sometimes these words have the same meaning, but in general use "import" to describe the thing being imported (This CD is an import.) and "importation" to describe the act of importing something (The importation of CDs into our country is a small business.)
In this case, I'd prefer "imports" if we're taking about the actual diesel. But if we're talking about the act of importing diesel, then "importation" (maybe taxes and tariffs associated with importing are part of the high expenses).