Can you please tell me what to do if I need to top-up on motor oil in an emergency, but a filling station only has synthetic oil available, while my car’s engine is running on mineral oil? Is it advisable to top-up a mineral oil with a synthetic oil resulting in a mix? What is the ratio to which the two can be mixed without harm to the engine? I have noticed that some of the motor oils on sale at supermarkets are mineral, semi-synthetic or fully synthetic.
N VERMAAK
Edenvale
Answer: In short it is better to top up with any available oil (even if it has a different viscosity grade) in an emergence rather than risking running with little or no oil. Obviously it is preferable to top-up with the same brand and viscosity if possible.
Oil consists mainly of carbon chains on a molecular level that are responsible for most of the core attributes of the oil. In synthetic oil, these chains are built-up using a specialised process by chemical engineers and they can therefore “tune” the characteristics of the oil to a fine degree. The result is that the statistical distribution of carbon-chain lengths is much narrower in synthetic oils. This allows synthetic oil to perform better at the extremes of temperature and load without breaking down.
Mineral oil is produced by refining crude oil. Therefore the carbon-chain lengths vary considerably more when analysed statistically. Under normal operating conditions, the oil will perform well, but at the extremes of temperature and pressure, some of the core oil qualities will start to deteriorate.
A semi-synthetic oil is essentially a blend between full synthetic oil and mineral oil. It has some of the qualities of synthetic oil but is not as expensive owing to the percentage of cheaper mineral oil content.
Therefore there is no problem topping up with mineral oil if you run full synthetic oil, but it will be a good idea to change the oil to full synthetic again if you expect maximum performance from your oil under extreme conditions. If you run mineral oil (as in your case) and can only top-up with full synthetic then you are actually improving some of the core qualities of the oil.
