Boiling oil to remove water is a poor mans fix, as compared to replacing the oil with new oil.
Oil contains numerous additives, many of them relatively unstable chemical compounds that are designed to combat, interact with, and capture contaminants in the oil - such as water.
The additives are designed to hold the water in an emulsion and prevent the water from contacting bare steel components, and thus creating immediate corrosion.
There are up to 7 major additives in oil -
- Anti-oxidants. Oxidation is the general attack of the weakest components of the base oil by oxygen in the air
- Rust and Corrosion Inhibitors
- Viscosity Index Improvers
- Anti-wear (AW) Agents
- Extreme Pressure (EP) Additives
- Detergents
- Dispersants
- Anti-foaming Agents
Heating the oil assists in destroying the additives, as at any temperature above 125°C (257°F), the relatively (chemically) unstable additives start to break down, losing their full lubricating and protection abilities. The oils basic lubrication abilities are greatly aided by the additives.
Additives can reach up to 30% of the volume of the oil, so we aren't talking small quantities here.
So, in heating the oil, you may drive off a quantity of water, but you've also degraded the additives via heat breakdown, so the remaining oil now has a lower corrosion protection and lubrication ability.
Of course, if you can add a moderate amount of new oil to the system at the same time as you heated the old oil, you're adding a certain level of new additives, which will assist in surface protection and improved lubrication.