Would this help a beardie with a sunken goopy eye?

No, this is a condition that needs antibiotics from a reptile vet after a culture is done to determine the bacteria or fungus that is causing the infection and the antibiotic or antifungal that will be effective against it. A beardie can go blind and lose hearing, or possibly have the infection spread to its brain, just like a human can if they have an infection in their eyes, ears, sinuses, or mouth/throat that goes untreated.

Does the syringe included have a needle for injecting the product into a feeder insect?

There is no needle included with the syringe.

Will this one work on a 8 year old bearded dragon?

Reptaid is designed for reptiles under 250 grams. Most adult bearded dragons usually weigh around 380 to 510 grams. If your bearded dragon weighs 250 grams or more, I would purchase Reptaid XL which is formulated for reptiles 250 grams and over.

Can I feed this to my red eared slider?

Yes. any reptile under 250g. Thank you for asking and feel free to contact us again with any questions. You may email us directly or call us. We are available Monday-Friday, 9 AM to 5 PM, MST.