Thank you for your question.  Clear photos of the stems, to see if there are brown spots would be helpful to diagnose the problem.  But I suspect that your plant has a bacterial infection described here:  https://pnwhandbooks.org/plantdisease/host-disease/lilac-syringa-spp-bacterial-blights

As you can read, pruning off the deformed leaves, pruning the entire plant for more air circulation, and protecting the plant from rain and frost in the spring may help in the future.  Since lilacs have 7 other known diseases (all of which have different symptoms than your plant), you can also submit plant samples to the OSU Plant Clinic for diagnosis:  https://bpp.oregonstate.edu/how-submit-plant-materials

Good luck!

I found the culprit.  While pruning I found the stem hallowed out and some larva in it.  Given the infestation I am planning on removing the plant.  So 2 question;

1.  I have azaleas next to the lilac.  Do I need to worry about them?

2.  Any suggestions for replacement plants that are not prone to the borer?

thanks

So sorry!  Here are the plants the lilac/ash borer infests:  "Lilac (Syringa spp.), ash (Fraxinus spp.), mountain ash (Sorbus) and privet (Ligustrum spp.) are the preferred hosts but other members of the olive family (Oleaceae) may also serve as occasional hosts."  https://ag.umass.edu/landscape/fact-sheets/lilac-borer-also-known-ash-borer

I would suggest that you read the following, which has ideas for native plants not susceptible to this pest:  https://extension.oregonstate.edu/catalog/pub/ec-1577-gardening-oregon-native-plants-west-cascades

Good luck!