Oil, the 'lifeblood' of our beloved motorbikes!
But what exactly does it do and why are there so many different types?

Oil is a lubricant and we shall start by looking at what a lubricant does in a motorbike and most specifically in the engine.

• Reduce Friction and Wear
• Clean away Swarf and Particulates
• Remove Heat
• Prevent Impact Damage

Lubrication and Friction
Friction is the force which opposes movement between two contacting surfaces. The action of a lubricant in separating these surfaces reduces both the wear and the force needed for the surfaces to slide past each other.

• Oiliness
  When oil is dropped or squirted onto a surface, its ability to spread out and cling to that surface is known as 'oiliness' (surprise surprise!). A good oil does this so effectively that it can only be removed with extreme heat, or chemical solvents.

• Viscosity
  This is the oils thickness or resistance to flow. A thick oil (high viscosity) can withstand greater loads as it will not squeeze out between two surfaces easily.

Viscosity Index and the SAE Classification of Oils
Oils were originally identified by a vague grading system such as 'Light', 'Medium' and 'Heavy'. But unfortunately these differed from one manufacturer to the next. The Society of Automotive Engineers (SAE) developed a specification for oils so that we could all compare oil from one manufacturer to the next. SAE numbers give no indication of quality, only a rough description of its viscosity at two set temperatures. The 'W' suffix states that the oils grade was set at -17.8°C instead of at 99°C. The lower temperature test is designed to check the cold weather performance of oils.

For critical usage be warned, an SAE20 oil and an SAE20W oil can often have very different viscosity's at room temperature. I have found instances of the variation being up to 150%.

Multi-grade oils
With straight grades of oil, an SAE 40 oil is thicker than an SAE 30 oil. Most modern engines however specify the use of multi-grade oils, such as a SAE 15W/50. This multi-grade specification means that at -17.8°C the oils viscosity will behave as a 20 weight oil, but when hot (at 99°C) it will 'apparently' thicken up to behave like a 50 weight oil. I say apparently because the oil will still get thinner as it heats up, but nowhere near as thin as a straight 20 grade would have done! (This is achieved by adding viscosity index improvers).

Other Classifications of Oils
So when buying oil from a supermarket and faced with two choices, one at £5 for 5 litres and the other at £25 for 5 litres and they are both SAE20w/50, so are they both the same? The simple answer is no, now read on!

There are now many classifications of oils - and I am not going to cover them all here!!!

API (American Petroleum Institute) Service Classification
This puts oils into 'suitable usage' catagories. I.E. Letter Designations: SG, SH, SJ, etc.

• SH - This oil classification came to the industry in 1992 to replace the SG oil and provide better protection against rust, oxidation, sludge, varnish as well as providing extended component life expectancy. It is currently still available, however it is obsolete for PCMO licensing. It can be used only with API CF, CF-2, DF-4 and CG-4 when displayed in the API service symbol and the C category appears first.

• This oil meets the current automotive manufacturer requirements for their newest lines of engines and will be used in 2,000 automotive engines. There were earlier versions used before the year 2000. The ISLAC grades are described as SAE 0W-20, 0W-30, 5W-20, 5W-30, 10W-30. These oils have improved overall oil performance while increasing fuel economy while providing emission systems protection.

MIL - American Military Specification.
DEF - British Ministry of Defence Classification.
ASTM - Engine oil use description.
ESE, SSM and ESP are Fords own Spec. Mercedes and Fiat now also have their own!

Here is an example of a typical modern engine oils specification description, as written on the rear of the container -

Castrol GTD Diesel - Premium diesel engine oil formulated for both diesel and petrol 4WD’s and light commercial vehicles. SAE 15W-40, API CG-4/SH, US Military MIL-L-46152.

At least now you will have an insight of what all the letters are about, even if you are still going to need a very comprehensive chart to cross- reference what all the specifications actually are!

Engine Oil Additives
To improve the various properties of lubricants, additives are included in the base oil. Their functions are as follows:

• Detergents - Improve engine cleanliness in the piston ring lands and other hot regions. They prevent coagulation of deposit forming gums and residues, etc. and keep them in suspension in the oil.

• Dispersants - Control cold sludging and carry finely divided combustion products in colloidal suspension in the oil.

• Oxidation and corrosion inhibitors - Reduce the rate of oil oxidation and control alloy-bearing corrosion.

• Anti-wear agents - Protect rubbing components against wear, especially valve train mechanisms where there is usually only boundary lubrication.

• Oiliness agents - Decrease the friction of heavily loaded rotating and reciprocating components.

• Anti-foaming agents - Reduce the tendency to foam caused by the engine churning action and continuous movement of the oil.

• Pour depressants - Are added to lower the solidification temperatures of oils.

• Viscosity-index improvers - Improve the viscosity-temperature properties, help control oil consumption and make cold starting easier.

Although each of these additives in the correct proportion for each specified task do a grand job, it is worth remembering here - that more additives means less oil! Most engine manufacturers speak out very strongly against adding 'extra' additives, so be warned as to the wisdom of purchasing an adding more additives than are absolutely necessary.

Slick 50
I have separated out this product as it is NOT an additive. Although this is a friction reducer and is applied by adding it to the oil initially. Unlike most additives, Slick 50 actually coats the metal surfaces which rub and thus the engine remains 'treated' long after the oil has been changed.

• EP oils
  Extreme pressure (EP) oils are used in gearboxes, where the gear-teeth in virtual point contact produce pressures high enough to destroy the molecular bond of a normal oil.

• Hypoid or HP oils
  Are used in Hypoid differentials on cars and lorries, where the shearing action of the sliding hypoid gear teeth actually tear normal oil apart. Modern EP oils are now so good that some will also be suitable for this use.

• Turbocharged engine oil
  The temperature that turbochargers run at is so high that the heat will destroy most of the properties of a conventional engine oil. Hence oil suitable for Turbo's must be used with turbocharged engines. NB: For the same reason, it is important to change the oil of any engine that has suffered serious overheating through say loosing its coolant.

• Diesel engine oil
  Diesel engines have special lubrication requirements due the fuel which they burn. These oils have a different balance of additives to help prevent sludging and also corrosive effects from the by-products of the fuel burning etc.

• 2 stroke engine oil
  Two stroke engines rely on a very sparse oil mist to lubricate the piston, rings and bearings (this is known as 'Boundary lubrication'). Due to these extreme conditions the oil used needs to have an outstanding ability to cling to surfaces, and also have great strength even as a thin film.

• Running in
  All moving machined components such as the many in an engine, need to bed in, or be run in. This happens when all the microscopically small rough edges are worn off to a much smoother finish, which eventually allows engines to develop their maximum performance without harm. Running in normally happens within the first 300-500 miles. But modern high performance synthetic oils are so good that engine are virtually impossible to 'run in' within a sensible time frame. It is for this purpose that special running in oils have been developed. These oils are only in the engine for a short period and have many additives changed or precluded to suit.

To high an oil level can create crankcase pressure build up, which may result in damaged gaskets and seals. Surplus oil can get past the rings and burn, thus causing pollution and high emissions. The rings could also stick in due to 'varnishing' problems.

Note: Motorcycles which share their engine oil with the gearbox and clutch, are extremely prone to oil contamination! 

Good engine design helps to reduce these problems and the oil filter also works hard to stop any harmful 'nasties' from circulating in the lubrication system. But never the less it is wise to change the oil and oil filter regularly. In most recent years service intervals have been extended greatly, but this is often to prove how cheap certain new models are to run!

My best advise is to treat manufacturers engine oil service intervals as an absolute maximum. Consider what use you put your bike to and compare it to what 'MR Average' with a new bike is expected to do. If you race - change it often. Serious racers perform an oil change after every race. If you park your bike up for long intervals and drive few miles, change your oil far more frequently than the stated mileage interval.

Remember - oil and a filter is relatively cheap and engines are VERY expensive. Read what the manufacturer recommends and check that what you buy meets that specification!

Happy biking and until next month Saludos - Adrian Harris

(This article is offered as an item of interest only and Bikers Website and the author take no responsibility for any misinterpretation by any reader resulting harm or damage to either themselves or any equipment.)

Article Copyright ©  Adrian Harris 02/04/02. Not to be reproduced without prior permission.

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