Viscosity depends on the intermolecular bonding, hydrogen bonding and length of hydrocarbon chain. In crude oil there are many natural compounds present in the oil which increase the viscosity. There fore purification of oil that is refined oil has less viscosity than natural herbal oil.

Emulsifying and making suspension may help. CMC, phosphatidylcholine, egg yolk may be used. 

BY DILUTING WITH THE OIL THAT ARE VERY THIN

You can use degumming methods.

It depends on which herbal oil it is and check due to which reason viscosity is higher. Then only we can decide for how to reduce its viscosity.

There are many Hair Oils in market, most of them are thin watery in nature, the oil prepared as per classical ayurveda methods are not like that, why?

As an expert in Ayurveda dear Prof. Lavekar please tell us whether viscosity has any bearing on the Ayurvedic property of the herbal oil or consumer preference or marketability or any other specific use or parameter/metric.

Dear Sir BRRR, I am a Formulation Scientist associated with few Ayurveda Herbal New products manufacturers, some of them insisting for thin nature oils in view of marketing, no doubt such oil is not having potent therapeutic action.  

Thank you. I am not an Ayurveda expert but I think in Youtube there are several demonstration videos on the preparation of diverse herbal oils. The ratio of ingredients, the repetition of certain types of extractions, the methods of concentration etc. may have a bearing on the viscosity of the herbal oils. If therapeutic value is not of importance and cosmetic value is of more importance, the extraction procedures may be modified. By removing certain chemical ingredients coconut oils which do not stick to hands have been made and are being marketed.

In ayurveda normally Til oil is used and definitely therapeutic value is important. People are also like cosmetic nature with therapeutic value. Coconut oil is also good and used. To reduce viscosity herbal composition cannot be changed. Only diluent like mineral oil may be added to some extent. Otherwise refined oil should be used. 

Thanks, Dr. Lohar but adding mineral oil or using refined oil will reduce the efficacy of the oil. In Ayurveda there is no separate cosmetic group, every product is of therapeutic value.

In my view the less viscosity oils -thin hair or massage oils available in market may not be that effective, a matter of debate. 

I fully agree with Dr Lohar that to reduce viscosity herbal composition cannot be changed. Prof Lavekar, you can  use Jojoba oil having less viscosity if you would like to reduce viscosity.  It has high skin penetration value as compare to other vegetable oil. It is exception in the field of vegetable oil as chemically it is non- glyceride but  liquid wax.