Clarifying or volumizing shampoos are generally best for oily hair. These formulas often contain ingredients that remove excess oil and buildup without weighing the hair down. Look for shampoos labeled 'oil control' or 'balancing'.

To reduce oily hair, avoid over-washing, as this can stimulate oil production. Focus shampoo on the scalp and use conditioner only on the ends. Also, minimize touching your hair and using heavy styling products.

Hair becomes greasy after a couple of days due to the natural production of sebum by the scalp's sebaceous glands. Factors like genetics, diet, hormone fluctuations, and product buildup can also contribute to faster oil accumulation.

Dermatologists often recommend shampoos containing ingredients like salicylic acid, tea tree oil, or sulfates (used sparingly) for oily hair. These ingredients help to cleanse the scalp and remove excess oil. A dermatologist can provide a personalized recommendation based on individual needs.