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).

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)

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.

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.

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.

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.

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.

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.

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.

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.

API stands for Americam Petroleum Industry and GL stands for Gear Lubricant, see below for their definitions:

API GL-1 Straight mineral oil.
API GL-2 Mild EP for worm gear oils.
API GL-3 Mild EP for spur and spiral bevel gears in axles and transmissions.
API GL-4 Medium EP, moderate severity hypoid gears, manual gearboxes.
API GL-5 High EP, all hypoid axles, some manual gearboxes.
API GL-6 Extra high EP, now obsolete.

Yes. Selecting the correct gear oil performance will provide the best protction to the components of the transmission.

SAE stands for the Society of Automotive Engineers. The SAE classification system is a way of defining how thin or how thick an oil is. This is known as an oil's viscosity. The classifications are listed here in order of increasing thickness: ASE 75W, SAE 80W, SAE 85W, SAE 90, SAE 140, SAE 250.

EP means extreme pressure and refers to the additive used in gear oils. This additive is designed to stop metal-to-metal contact taking place between transmission components. The EP additives are usually based on sulphure and phosphorous. These elements bond to the metal surfaces where there are points of extreme pressure and temperature, forming a sacrificial chemical layer. The sulphur gives gear oils their characteristic smell.

Yes, but beware, there are two kinds of synthetic gear oils available: polyalphaolefin (PAO) based and polyalkylene glycol (PAG) based. PAOs are basically a man made version of mineral oils (although with greatly improved properties) and can therefore be mixed with mineral oils. In fact, semi-synthetic products have mineral and synthetic (PAO) base fluids in them, so obviously, they must be able to mix. PAGs, on the other hand will not mix with PAOs or mineral oil. Utmost care must be taken when using this kind of product.

Hypoid is an abbreviation of hypocycloidal and relates to the geometry of the crown wheel and pinion arrangement usually on rear wheel drive cars and vans. The pinion is usually highly off-set to reduce propshaft intrusion into the passenger compartment.

Yes. When the power distribution between two drive shafts is no longer equal (usually due to the surface condition that the drive wheels are turning on, i.e. ice, mud) limited slip differentials are able to effectively lock the two half shafts, ensuring equal power distribution. When this limited slip differentials 'kick in', there are high shock loadings on the clutch mechanism that requires protection from wear and slippage. The use of incorrect oils can lead to clutch degradation and vibration.

Depending on the age, make and model, non-EP gear oils may be required for use in gearboxes and final drives. Certain designs contained a lot of phosphor bronze (copper containing) components that are sensitive to the sulphur-based extreme pressure (EP) additive. The sulphur attacks the copper and destroys the integrity of the meshing gear surfaces.

Certain designs do specifiy the use of an ATF fluid (automatic transmission fluid), but they should only be used where it is clearly stated by the manufacturer.

This will depend on makes and models, but very often the answer is no. Gearboxes, final drive, transfer boxes, etc. all have their own specific lubrication requirements. The specification of the oil required will be outlined by the design engineers, who will determine which type of oil will provide the maximum protection to the transmission components. It may certainly be possible to rationalise and reduce the number of lubricants used, but the magical single product may not be achievable.