Oil is not a moisturizer even though many people mistakenly believe that it is and some oils are also advertised as moisturizing when this simply isn’t so. The purpose that oil plays in your hair care regimen is that it works as a sealant.
Many of us complain of wetting our hair and moisturizing it only for the moisture to leave as soon as we seem to put it in our hair. This is where the use of oil comes in. After applying a water based moisturizer, the oil is used as a final measure to prevent the moisture you just applied to your hair from leaving.
How long your hair will stay moisturized depends on your hair type, the weather (whether its hot or cold), the type of moisturizer used, and the type of oil that you used. You also have to keep in mind the natural look and texture of your natural hair. Kinky textures may appear as if they look dry but when you touch it, it may feel soft meaning that your in good shape, its just how your hair looks.
Mineral Oil and Petroleum are both highly criticized in the natural hair community. However, even though they are cheap oils and there other oils that you can choose from, if it were used to seal moisture into your hair it would get the job done.
As far as using it on your scalp you may not want to use these products because the thick layer that they create to prevent moisture from getting out may clog your pores. If you have a moisturizer or a styling product that works well for you and it contains mineral oil or petroleum then you do not have to toss it just because they contain these oils.
Just remember that a quality moisturizer will list water as its first ingredient and if it contains mineral oil or petroleum then it should be further down on the ingredient list.