What is an ashless oil?

When talking about lubricants, ash refers to the metallic compounds remaining in the oil after the hydrocarbons have been calcinated. This ash is mainly derived from the chemicals used in anti-wear additives and detergents, as well as solids – including calcium, zinc and molybdenum.  But these chemicals, which are frequently used in engine oils and hydraulic fluids, also have a number of disadvantages. They can lead to the formation of deposits in humid environments or at very high temperatures and can result in high levels of eco-toxicity for the environment – particularly when heavy metals are used.
Ashless industrial oils are therefore lubricants that are formulated without metallic additives, and which are mainly used for the following purposes:
– circulation oil used in a humid environment
– environmentally acceptable oil 
– Food grade oil

The definition is considerably different as far as engine oils are concerned: an "ashless" engine oil actually has a limited ash content (less than 0.01%). This is so as to prevent the pollution control systems from getting clogged up or poisoned.