you can physically inject anywhere you want, inject your eyeball if it really suits you. but if your asking if its a good idea, well you don't require a phd to figure that one out.
you can't gain added size by injecting into your penis.
site injection is a myth. its only bigger from the swelling of the injection.
Synthol, im sure may people have thought of it. but synthol works by your muscle fibers holding the oil in place. 30% of the synthol os broken down and the remainder stays in the muscle fibers and breaks down slowly within 3-5 years