You can also use rosemary essential oil or you can make a honey rinse by mixing it with some water and putting it on the scalp. I have tried both of them because my hair gets oily and I can't use castor oil.
They all just help to unclog follicles so hair can grow. My hair grows really slowly so I was looking for a way to speed it up and that is how I learned about these things.
Coconut oil also works to help hair growth. I use it as a pre-shampoo. Then you can wash it out with a shampoo and it won't leave your hair greasy, but using any oil as a pre-shampoo if you want to be able to wash your hair more frequently, is a good idea. It will help prevent dryness caused by shampooing without depending on conditioners with chemicals. I have given up conditioner completely. I just use a pre-shampoo with some coconut oil every now and then.
I think coconut oil is also supposed to prevent hair from getting gray too.
Although, if you can use the castor oil, I would think that would be the best choice because I have been using it on my eyelashes for the past couple years and it does really help them grow and become thicker, so I would imagine the same is true for hair growth. I just can't use castor oil on my hair, unfortunately.
Castor oil is also very thick so it might be good to thin it out a bit with either olive oil or Jojoba oil if you are going to try to use it on your scalp. Otherwise, it is basically the consistency of honey by itself and difficult to work with.
You might want to also consider dietary changes. Some of the most effective beauty treatments are internal. Flax seeds are good for healthy hair. You can also add coconut oil to juice or I put it in my coffee because coconut oil is solid at room temperature.
@Aero you might want to try beer or champagne for the oil on your head. You don't have to leave it on very long. Maybe like 5 minutes and it will help remove oil. If you use champagne, it will even lighten your hair a bit too. Both work pretty well in my experience, but it can give you a bit of a contact high.