Yes for butterflies and moths. You can use the Caterpillar Host Plant Database to search for which species use it. I've never seen any caterpillars on it.

You will need to look up the latin name. When I did the search I had to delete a few letters. If your initial search doesn't work, try deleting a few letters at a time and search again. I also specified USA as a location.

I searched on: Syrin the name for Lilac is Syringa. There were 27 species, mostly moths. 3 swallowtails.

Here is the search for Lilac as a host plant.

Here is a link that might be useful: Caterpillar Host Plant Database

The seed needs cold stratification, if I'm not mistaken. Has it been exposed to cold at all?