After day 5 from your FUE the donor is considered healed and you can treat it normally. In fact you have to go back to normal washing and hair care as you have done before your hair transplant. You may need shampoo longer at this point if you still have scabs and by cleaning the area the scabs will be gone and that is how you can help you itchiness. Using oil at this point is useful.
Talk to your doctor. They likely have some lotions etc. in mind specifically for this purpose, that wouldn't affect the healing of your scalp. Once you're at the two week mark and all the scabs/crusts are gone, then you should be able to freely use whatever type of lotion you prefer.