What are the problems if valuation of all the currencies are equalised to a single monetary unit?

I think you are confused. Are you talking about using one currency through out the world? or using existing currencies of different countries, equalising their value only per monetary unit? The value of crude oil or other commodities do not depend on the currencies in which they are transacted. Even when you have through out India, Indian currency, Rupees, with the same value per monetary unit, prices of the same commodity differ from one place to another.