I think that some varieties are more resistant than others (when you look around, you can see some with more and less of the affliction). But I haven't seen a list specifically of powdery mildew resistant varieties. I love lilacs, and have a handful. It's been a wet year up here, so the mildew is pretty bad this year. In a dry year, it's not so nasty. Sorry I can't name any truly resistant ones!

Laurel

Mine all have powdery mildew, and my big monster one is usually impervious to everything. But not this year. I'm sure some varieties are more resistant than others, but no idea which offhand. And, like I said, mine that are normally unaffected look like they're pretty bad this year.

I think you're seeing mildew on all the lilacs because this has been an extremely wet summer accompanied by extreme heat in July and now it seems we may have a very wet fall too.
The ground is saturated. It's been a bad year for the lilacs and my tomatoes. Fighting mildew everywhere.