Welcome to ATP, Bruce. Hope we can help you. A couple of things I remember about growing lilacs from when we lived in Utah:
They really like to be in full sun, and yes, a basic shrub/plant sustained release fertilizer (something that says "lasts 3 or 4 months" on the label) applied as they start to leaf out in the springtime will help them a lot. Lawn fertilizer doesn't work, as it's too high in nitrogen. Be sure to sprinkle the recommended amount all around the shrubs, out beyond the perimeter of the leaf canopy.
Oh, and this is very important! You must not prune lilacs until after they have finished blooming in late spring. They bloom on the new growth, so if you prune after they bloom, then they will grow lots of new bushy, healthy growth over the summer, and bloom generously the next spring on that growth.
Most plants benefit a lot from a good layer of mulch over the root zone, as well, to moderate temperature, help maintain soil moisture and keep down weeds. That was really important in the high desert where I used to grow lilacs.
