Frequently Asked Engine Oil Questions

1. Can I top up mineral oils with synthetic oils and vice versa?
Yes. The synthetic fluids (polyalphaolefins) used in engine oils are man made versions of mineral oil and are therefore compatible with mineral oils. In semi-synthetic formulations, mineral oils and synthetic oils are  part of the same formulation.  The only downside here is if you top up expensive synthetic oil with mineral oil  you will be effectively diluting the advantages that such an oil will provide (good cold start, good high temperature performance and longevity).

2. What is bore glazing?
Bore glazing is  a condition that usually occurs during the first critical hours/miles of an engine's life.  If an oil of too higher quality is used and the engine is not subjected to the correct loading (light loading is particularly bad), the honing marks become filled, making the bores smooth. With nothing to retain the oil in the bore, it will begin to disappear down the exhaust pipe. Symptoms tend to be high oil consumption, smoking and poor compression.  It is possible to cure by adding a glaze-busting additive to the fuel.  If this fails, re-honing may be required.  (See also the bulletin on Running In New or Re-built Engines)

3.  What is bore polishing?
When oil burns in the combustion chamber,  it creates ash that is very abrasive.  This can be particularly bad where high  ash products are used in engines used in high temperature, stressed conditions.  The ash effectively polishes  away the honing marks and leaves nothing to retain the oil in the bore.  Once again, as with  bore glazing, the symptoms are high oil consumption, smoke and reduced compression.

4. What is detergency and dispersancy?
Detergency refers to the oils ability to keep engine components clean, particularly those in the hotter parts of the engine (pistons, rings, valves, etc.). The additive is referred to as a detergent.  Dispersancy refers to the oils ability to keep solid contaminants (i.e. soot, combustion debris, etc.) in suspension.  This is necessary for two main reasons:  firstly, it delivers the contaminants to the filter where the bigger particles are removed and secondly, it ensures that all the smaller particles flow out when the oil is drained, leaving the engine clean.

5. Why can't I use high detergent/dispersant oils in my classic car?
Usually this will depend upon the type of filtration used. If a simple mesh strainer is used on the pump inlet, low detergency/dispersancy oils are a must.  As previously described, dispersants keep all the 'rubbish' in suspension and allow it to circulate.  Simple mesh strainers aren't efficient enough to remove it and  so it continues to circulate causing damage.

6. What's the difference between a multigrade and a monograde?
The main difference between these two types of engine oil is their fluidity at cold start.  Multigrades, such as 10W/40, 15W/40, 5W/30, etc., flow more easily and are therefore pumped round to the critical components much more quickly. Historically, the 'W' stands for winter. As well as this important feature, multigrades also have to provide a protective oil film at higher temperatures when the engine has warmed up.  Monogrades on the other hand provide a very good oil film at working temperatures, but their cold start properties are poor. To overcome this it used to be the practice to put a thin monograde, such as a SAE 30, in the engine during the winter and a heavier monograde, such as a SAE 50.

7. What are API and ACEA specifications?
API (American Petroleum Institute) and ACEA (Association of European Automotive Manufacturers). These are North American and European specifications respectively. They are compiled by the Original Equipment Manufacturers (OEMs) and are recommended by them in handbooks to ensure the correct quality of oil is used to protect their own particular engine designs.

8. What do the SAE Classifications mean?
SAE stands for the Society of Automotive Engineers.  This is a classification system used to categorise how thick or thin an oil is.  The classifications are listed here in order of increasing thickness:  SAE 10W, 20W, 30, 40, 50.

9. Is there a special oil for turbochargers?
These days it is more appropriate to ask," ... are there oils available that are not suitable for turbochargers?" , as nearly all modern lubricants will protect turbos in petrol and diesel engines.

10. In my car, if I use a synthetic product, can I leave it in the engine longer?
No. Always refer to the vehicles handbook or seek technical advice as far as oil drain intervals are concerned.  The oil drain interval will be assigned by the vehicle manufacturer and will take into account filtration, fuel type, power output, etc.  If the oil is mineral based or synthetic it should still meet the manufacturer's requirements and as such cannot be used past the recommended oil drain interval.