I use a variety of silicone serums and a few sprays. I like Moroccan Oil, Chi Silk Infusion, Got2Be Glossy, Cowboy Magic, LOMA pearitin Repairative Serum; also (sprays) I like 'It's a 10 Leave-in' and Citre Shine shine mist.
To be honest, I think most serums (drugstore or high end) will be good for good quality extensions. The trick is how they are applied and how often. they should be applied over some type of leave-in conditioner. IMO they should not be applied daily unless hair is being washed daily.