Techniques: The Oil Truth

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There are probably more myths and misconceptions about oil and lubrication than any other subject in motorcycling. Gerry Bristow attempts to set straight some of the errors that masquerade as facts…

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What oil has to do

The primary function of oil is to prevent metal to metal contact by introducing a film of oil between the various bearing surfaces, which have many different requirements as far as oil is concerned. Taking a few examples found in a motorcycle engine, there are roller bearings which are virtually frictionless and therefore require little lubrication in the normal sense but benefit from the ‘cushioning’ effect of the oil film. Roller bearings usually fail by compression fatigue. As the bearing rotates, the track is being continually loaded and unloaded as the rollers pass along its surface. In a process known as galling, flakes of the surface will eventually separate. Ball bearings suffer the same problem but more so because of the higher loading for a given size and they also have a degree of sliding contact. Ingress of foreign matter in either type of bearing is fatal.

More severe conditions occur between the cams and cam followers, where there is high and constantly fluctuating loading and between piston rings and cylinder and valves and guides where the temperature is high. All these situations require a flow of oil rather than high pressure, whereas plain big end bearings and bushes also require pressure to maintain the oil film.

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Other important functions are heat dispersal and the removal of the products of wear and combustion which are either collected by the oil filter, if fitted, or carried in the oil as microscopic particles too small to cause problems. If they are large enough to be deposited in the sump it’s probably too late to be worrying about what kind of oil you’re using!


This is probably the most misunderstood of all commonly used terminology. I’ve always thought that it’s an unfortunate choice of name because it conjures up visions of a powerful cleaning action like some household cleaner, whereas all it actually does is prevent microscopic particles from sticking together and forming the dreaded sludge. It won’t dislodge great dollops of the stuff and block up the oil ways as is sometimes claimed and anything small enough to pass through the filter is too small to cause damage to bearings etc.

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The six most important properties of engine oil

These are: viscosity, viscosity index (VI), flash point, pour point, % zinc dialkyldithiophosphate (usually referred to as zinc for obvious reasons), and % sulphated ash.

ViscosityGoes in clean, comes out dirty...

Viscosity is the measure of thickness of the oil. Too low a viscosity and the oil may shear and will lose film strength at high temperatures. If too thick, it will not circulate at low temperatures. The SAE ratings given to oils are numbers assigned by SAE International, formerly the Society of Automotive Engineers. If you look at the graph you will see that they represent a range of viscosities. The ‘w’ indicates that the viscosity was measured at 32 oF (0 oC) a hangover from the days when a thinner oil was used in winter. Of course no one does that nowadays do they?

It’s also apparent that two oils labelled 20w-50 can have quite different characteristics according to where in the range their viscosity falls. In the ‘good old days’ Duckhams Q20-50 was in the middle of the 20w band and the 50 band. Some others were not so honest and a 15w classification was introduced to give the lower viscosity rating more credibility — or was it so that the major oil companies didn’t have to label their oil 20w-50, which to most people meant Duckhams?

Another interesting thing to note is what happens at really high temperatures. The 20w-50 oil retains its viscosity much better than a 50 monograde and temperatures in an engine can reach 350 to 400 F, in the piston ring grooves for example.

VI and how multigrade works

Multigrade graphTo produce a multigrade mineral oil, polymers are added to a light oil, say SAE 20. At low temperatures the polymers are coiled up and do not restrict the flow, but as the temperature rises they unwind so that at 100 oC (212 oF) the viscosity is equivalent to SAE 50. There is more than one way of achieving this result. The best quality base oils have a higher VI, which is the rate at which the oils’ viscosity falls as temperature rises within a given temperature range. This means that less polymer is required to achieve the same result. As it’s the oil that does the lubricating, not the polymer, this is obviously preferable and as the polymer also degrades in use the less present to start with the better.

A good quality engine oil will have a relatively low level of polymers and some synthetic oils have such a high VI that they don’t need any. Generally the lower the viscosity of the base oil, the more polymer it requires. 10w-40 needs more than 20w-50 for instance, and polymers can shear and burn, thus creating deposits which can cause valves and piston rings to stick. A good quality oil will typically lose up to 20% of its’ viscosity in 1000 miles running, the rate of loss then slows and the life of the oil is about 3000 miles in normal use.

Viscosity rating chart


This is the temperature at which combustible vapours are given off. The lower the flash point, the greater the tendency to vapourisation loss at high temperatures and to burn in contact with the hot piston crown, valves etc, resulting in varnish-like deposits. Flash point is measured in degrees F, the higher the better, and needs to be above 400 oF to limit consumption.

Pour point

This is given as 5 oF above the temperature at which the oil shows no movement at the surface for 5 seconds when inclined. This is very important in winter and additives called pour point depressants are used to increase fluidity at low temperature.

% zinc

Make sure you get the right pour point.This is the amount of zinc used as an extreme pressure anti-wear additive. This only comes into play as a last resort when the oil boundary layer has broken down and metal to metal contact occurs. Hopefully this only happens rarely, but when it does the zinc compound combines with the metal to prevent scuffing and wear. About 0.11% to 0.12% zinc is the optimum. More is not better, it just lasts longer and excessive concentrations can actually lead to increased wear, the formation of deposits and sparking plug fouling.

% sulphated ash

This is how much solid material is left when the oil is burned. High ash content forms more sludge and deposits in the engine and causes increased valve wear.

Reading the label on the can

All of the above values are obtainable from any reputable oil company, but only tell part of the story. The type of base oil used, the type, quality and proportions of additives used are just as important. If you read the small print on a can of engine oil you will find a number of specifications which the oil is designed to meet. The minimum to look for on mineral oil is API SG/CD where API is the American Petroleum Institute, S means it’s for petrol engines and G indicates the type of additive package, CD means it’s also suitable for some diesels.

This is important if you use own brand oils, Tesco, Halfords etc. They’re OK but the problem is that they’re not always the same — the business is put out to contract periodically and whoever quotes the best price supplies the next batch. They’re supposed to be to the same spec, but who knows?

Synthetic oils

CB350 not shown.(Only kidding, Paul)These have been developed over the last 30 years initially to cope with the extremes of temperature experienced in high altitude jet engines and space vehicles. They are made from synthetic base fluids — polyalphaolefins (PAOs) and esters — plus an additive package. PAOs are made by chemically inserting molecules of ethylene into carbon chains of uniform length and shape that remain liquid and slippery under a wide range of conditions. These are combined with esters, which are compounds formed by the reaction of alcohol and certain acids.

Synthetic oils have a number of advantages over the old dead dinosaur juice and, used correctly, virtually eliminate wear caused by friction. Partly because they use little or no polymers, they retain their viscosity much longer, have a higher flash point (typically around 500 oF), a lower pour point [around – 60 oF] and, because the esters molecules carry an electrical charge, they cling to the metal surfaces in the engine indefinitely and provide lubrication on start up. There are also none of the waxes and other unwanted constituents found in mineral oil. Unfortunately these desirable qualities come at a price. Synthetics are up to three times the price of conventional oil but can also last three times as long, so the long term cost is not so different, in any case the annual cost of oil is small compared to the total running costs.

If Norton recommended 50 grade oil for my dads’ 1932 16H, it was because that was the best available at the time. It’s not a reason for not using the best available today, and by 1970 they were recommending 20w-50. All the evidence points to the fact that the correct viscosity range for air cooled bike engines is provided by 15w/50 or 20w/50 oil, ( Duckhams Q20-50 was specifically designed for motorcycles, it was only by accident that it was found to benefit cars, but that’s another story) and the best and longest lasting protection is provided by synthetic oil.


First, if you don’t already have one, fit a proper filter. If you’re worried about maintaining the original appearance, you can hide it in the toolbox.

In all the independent tests I’ve seen nothing has ever beaten Mobil 1. Mobil produce a 20w-50 synthetic oil for bikes called ‘Mobil 1 V-Twin’ which would be my first choice if I could get it, but it’s only available in the US and they don’t plan to introduce it in the UK at present. In the UK they sell ‘Mobil 1 Racing 4T’ which is 15w-50 and is suitable for all air-cooled bikes. It’s a pity about the name, which sends the wrong message. There is also Mobil 1 15w-50, which is intended for cars, and is to SJ specification. As most of the motorcycle specific oils on the market are simply relabelled car oil, I’m not bothered about this and after using it for some 5 years in my Commando I’m very pleased with the results.

If you wish to change from one kind of oil to another it’s not necessary to clean out the engine, unless it’s dismantled of course, and don’t use flushing oil as it’s usually mostly paraffin (kerosene) and no matter how careful you are, some of it will be left in the engine and will contaminate the new oil.

Running In

If you’ve just rebuilt your engine and decide to use synthetic oil then don’t use it whilst running-in, it will take forever. Better to use a good quality 20w-50 mineral oil for the first 1000 miles or so and then change to synthetic.


This is a whole separate subject, but can be dealt with very briefly. Don’t! They are designed to make money, not to improve the performance of your oil. There’s nothing wrong with that of course if they’d write it on the tin instead of the fictitious claims of less wear, better fuel consumption, success with girls etc.

We all want to believe in magic; it’s a pity you can’t buy it in a tin.

Gerry Bristow spent most of his working life in the oil industry, with Duckhams, BP and others.

Oil. With our Commando, inside the engine is the last place you look for it

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