Feature: Oils ain't oils

What oil does is pretty simple. Oil is slippery so it's used in an engine to reduce friction between metal surfaces, which in turn prevents wear. From here things get confusing, because there's a squillion different oils available.

Luckily for us bike riders, there are really only two types we have to deal with: gear oil and engine oil. There are two sources of oil. It can be found in the ground (mineral oil), or it can be man made (synthetic oil). Mineral oil is purified, but it's still a mixture of several hundred chemicals. Mineral oil is more prone to oxidation and varnishing engine components under extreme load.

Synthetic oil has a strictly controlled composition. Synthetics are much better at resisting oxidation and inhibiting varnish deposits, but are expensive to make

Some smart cookies mixed the two together to create semi-synthetic or synthetic-blend oil. This reduces costs, but offers better protection under load than a straight mineral oil.

The general rule is that by using a full synthetic oil you can leave it in for much longer than a mineral and get the same protection. However, modern thumpers are an exception to this because of how their engines operate (I will explain this later).

Oil works better when mixed with additives. Different additives do different jobs. Some improve lubrication, others stop foaming and so on. Oils formulated for specific purposes have particular additives, and this is the real secret to knowing what oils are safe to use for each purpose. Many oils can be used safely for more than just their stated purpose. Knowing this can get you out of a pickle when it all goes pear shaped. Check the sidebar opposite for a list of common oil additives and their functions.

GEAR OIL
You'll only have to use gear oil if you ride a two-stroke or have a thumper with separate engine and gearbox compartments. Gear oil is cheap. Most gear oils are mineral oils but there is an increasing amount of 'semi-syns' also available.

Gear oil can be mono-grade or multi-grade. Mono-grade oils only have one number in their grade, like SAE40. Now is a good time to explain these numbers. This is a grading set by the Society of Automotive Engineers (SAE). SAE grades for gear oil and engine oil are not the same. An SAE50 gear oil is roughly half the viscosity of an SAE50 engine oil.

If you have only one number, you have a mono-grade oil that thins as it gets hot - these are quite acceptable (and cheaper) when the operating temperature doesn't vary greatly.

However, everyone knows that oil thins as it gets hot and this lessens its protective powers, so many oils have viscosity enhancers. These start all curled up when cold and expand when they get hot, thickening the oil. Such oils are called multi-grade oils and are recognised by having two numbers in their grade denomination, such as 10W/40.

These numbers relate to the viscosity of the oil and the higher the number, the more viscous (thicker) the oil. Thicker oils offer better protection at high temperature, but can have issues with starting and lubrication in cold conditions. The first number is the viscosity of the oil at zero degrees Celsius - this is called the winter viscosity, hence the 'W'. The second number is the viscosity at 100 degrees Celsius, or the operating viscosity. What this tells you is that a 10W/40 oil is a '10 weight' oil, but with the effect of the viscosity enhancers it lubricates as well as a 40 weight oil at operating temperature. Simple, eh?

The distinguishing feature of any motorcycle gear oil is that it has no friction modifiers. Friction modifiers are a no-no for wet clutches. Wet clutches are found on most bikes and are immersed in oil. The friction modifiers would stop the clutch plates from grabbing, and soon you would be going nowhere.

ENGINE OIL
This is slightly more complicated. Two-strokes burn the oil, whereas four-strokes pump it around then re-use it. Two-stroke oils are mono-grade, but what's important is that they mix with the fuel. This is a definite case where synthetic is better than mineral - it burns cleaner and provides better lubrication under extreme load.
There are grades of two-stroke oil too; Japan has developed a grading system specific for two-stroke oils, see the sidebar for more details.

Four-stroke engine oil is where things get complicated, and where the canny buyer can get optimum protection for the best dollar value. Just like their two-stroke cousins, four-strokes run a wet clutch and friction modifiers are a no-no. This is the only thing stopping you using car engine oil. Car oil without friction modifiers will work just as well in a motorcycle. Any oil labelled "energy saving" or "low friction" will have friction modifiers. Generally oils of 10W/40 and higher won't have them, but read the label before you drop in it with your clutch.

But there is one issue that we haven't looked at, and that's fuel dilution. Modern thumpers are high output compared to their predecessors. They do this by lots of clever engineering.

The most important aspects here with regard to oil are short-skirt pistons with single compression rings, which drastically reduce friction. The side effect of this is more fuel mixture can squeeze between the piston and cylinder wall, escaping into the bottom end, where it subsequently dilutes the oil.

This is the reason that thumpers need frequent oil changes. The oil is thin enough to be useless long before it has had a chance to degrade from heat and oxidation. This is one situation where a full synthetic won't offer its full benefit.

Two strategies are helpful here to get best protection for your bike, without you needing to sell your children for scientific experiments. You don't get the benefit of a full synthetic oil, so buy a semi-synthetic. They are a fraction of the price and for the time they are in your bike they will do a good job. To help combat the dilution buy a heavy weight oil. The heaviest I'm aware of are 15W/50 and 20W/50, and these are what I use in my much loved Gasser.   

WHAT THE OTHER NUMBERS ON THE LABEL MEAN
 Many countries have adopted their own oil grading systems, adding to the confusion when you read the label. Hopefully this will shed some light on what the numbers and letters mean. The two most common are from USA and Japan.

API (American Petroleum Institute):
The 'API' code uses a two-letter grading system. The first indicates the type of engine it's for: petrol - 'S' (spark ignition), diesel - 'C' (compression ignition). The second is oil quality: A is the lowest quality, J is the highest quality.

JASO (Japanese Automotive Standards Organisation):
 JASO two-stroke oil specifications are based on the oil's lubrication properties, detergents, initial torque, smoke formation and unburnt oil deposition. There are three grades for two-stroke oils: FA, FB and FC. FA is the lowest grade, FC is the highest.

JASO four-stroke oil specifications cover criteria relating to the oil's ability to resist clutch friction and slippage, and to prevent wear and corrosion of gearbox internals. The two grades for four-stroke oils are MA and MB, MA being the relatively higher friction oil.