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This article was first published in 2002.
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When you walk into your local auto parts store it's easy to be overwhelmed by the number of different oils and fluids on offer. DOT X, API, 15W/40, semi-synthetic - what does it all mean?
Engine Oil
Nearly all engine oils these days are multi-grade - this oil has a 'split personality' that makes it effective over a wide temperature range. In general, oil thickens when it's cold and thins as it heats up. When it's cold, it is difficult to pump around the engine. That costs power and for a time starves some components (usually valve gear at the top of the engine) of lubrication. When cold, multi-grade oil is quite runny, which allows it to be pumped more easily to vital engine components when the engine is first started. As the oil warms, however, it doesn't thin-out as much as old-style mono-grade oils, which were specified as a compromise between cold and hot performance.
The single most important part of engine oil selection is viscosity.
The SAE (Society of Automotive Engineers) has a system for measuring and grading oil viscosity. For example, an oil labelled SAE 20W/50 indicates it has 20 viscosity rating when cold and a 50 rating once warmed to normal operating temperature. The higher the viscosity number, the thicker the oil. Incidentally, the W in the above description stands for Winter, and indicates the lowest viscosity of the oil when it's cold. Although mono-grade oils are virtually extinct today, the current viscosity system references the behaviour of multi-grade oils to that of mono-grade oils. A 20W/50 oil has the viscosity of a 20 oil when cold, but the viscosity of a thicker 50 weight oil when hot. It doesn't thin out as much when temperatures rise.
Many of today's cars use 15W/40 or 20W/50 multi-grade oil, but the only sure-fire way to find out the appropriate viscosity for your car is to consult your owner's manual. Many cars manufactured in the last decade specify thinner oils that feed overhead cams quicker on start up and allow turbos to spool up quicker. Lower viscosity oils help performance by costing less power to pump around the engine. A secondary benefit is less fuel consumption - important for manufacturers chasing compliance with ever-tighter emissions regulations.
In addition to checking viscosity, it's important to check that oil meets the most recent API standards.
API stands for American Petroleum Institute. The API monitor various types of oil testing, including protection against sludging, ring sticking and varnishing, viscosity change and how much a given engine wears when subjected to stop-start conditions.
In 1972 the API standard was designated SE (API SE), and since then we've moved on to SF, SG, SH and - most recently - SJ. The progression has seen many improvements including reduced amounts of stray chemicals such as phosphorus, which improves cat converter life. The latest standard - SJ - offers the best quality protection for all engines.
Note that the S prefix relates to petrol engines, while a C prefix applies to commercial diesel engines. Some oils can pass both API's petrol and diesel tests.
Another important consideration is whether to use mineral or synthetic oil.
Mineral oils are based on the direct distillation and separation of crude oil pumped form the ground, while synthetic oils are synthesised from pure chemicals (although it must be said - some of which are derived from crude). There are also part synthetic and semi-synthetic oil blends on the market. Synthetic oils are complex and are said to offer longer service life, reduced oil consumption, better cranking and pumping characteristics at extremely low temperatures and the potential for better stability at high temperature. There is usually no problem switching an engine from mineral to synthetic oil - again, though, we'd advise checking your owner's manual for manufacturer details.
Of course, it's never advisable to mix engine oils - the resulting combination may cause problems due to incompatibility between some of the chemicals present (such as cleaning additives provided by the manufacturer).
Manual Gearbox/ Differential Fluid
SAE viscosity standards also apply to gear oils. A viscosity number higher than 60 indicates the fluid is designed to protect gear type components - as found in conventional gearboxes, manual front-wheel-drive transaxles and differentials. The most common grades are 75W/90 through to 85W/250.
In general, gearbox oils are required to be far thicker than engine oils. This is due to the extreme pressure between the gear teeth in a gearbox - a thin oil would be squeezed out from between the teeth of the gears, allowing abrasive metal-to-metal contact. However, some manufacturers specify engine oil in the gearbox, generally only in low-power cars.
The viscosity of gearbox lube affects how quickly the gear synchro ring engages. From a performance point of view, a thinner lube is preferable because it allows rapid synchro action, and therefore a less-baulky gearbox, but - once again - car manufacturers specify the appropriate viscosity gearbox fluid for their vehicles. Be warned - deviating from the specified fluid grade may result in gearbox damage and invalidate any warranty.
Both manual gearboxes and differentials pose similar stresses on gearbox lube and, therefore, front-wheel-drive transaxles (containing the gearbox and diff) don't usually require any special fluids. Vehicles equipped with a limited slip differential sometimes require a specific lubricating fluid.
Automatic Transmission Fluid
The most complex fluid used in a car is automatic transmission fluid. Auto trans fluid must serve as a power transfer medium, a hydraulic control fluid, a clutch fluid and a gear and bearing lubricant. It is also the auto transmission's cooling medium so must be capable of absorbing heat and carrying it to the radiator.
The most common transmission fluid used in modern cars is Dexron III, though some fluids are formulated to suit specific transmission models. Valvoline Type 95LE, for example, is developed specifically for the Australian-made BTR Engineering four-speed autos fitted to Ford's Australian Falcon and exported for use in other cars such as the Ssanyong Musso and some Maseratis.
It's extremely important to follow the fluid recommended in the vehicle's owners' handbook - it's not a good idea to 'try' something else! Also, as a rule, it's a good idea to drain and refill the transmission every 25,000 kilometres or 12-months.
Power Steering Fluid
Many power steering systems are happy to accept automatic transmission fluid; since similar viscosities and thermal properties are required. Valvoline's Dexron III fluid, for example, is suitable in most automatic transmissions and power steering systems.
In some vehicles a specific power steering fluid is necessary. Valvoline Formula F Power Steering Fluid is designed for Ford AU models, for example.
Coolant
Radiator coolant is available in two types - a ready-to-pour complete fill or a concentrate requiring mixing with purified water. The direction you should take is largely dictated by cost.
The reason for using coolant is its anti-freeze, anti-boil and anti-corrosion protection - particularly important for engines with an alloy head and/or block. In winter in many parts of the world, temperatures below the freezing point of water are routine. Water expands when it freezes and if allowed to freeze within an engine will quickly blow apart radiators and engine blocks. Alloy and iron - the materials many engines are made from - in contact with each other via tap water (that conducts electricity) begin corroding each other. The electrical currents present in any engine simply add to the chemical equation. Coolant must prevent cavitation corrosion of the water pump, crevice corrosion in joints and castings, formation of scale, soldered joint corrosion and general rusting.
From a climatic perspective, factory-fill coolant typically elevates the system's boiling point to around 127 degrees C, and lowers its freezing point to around minus 14 degrees C. Colder climates usually require a higher concentration of coolant.
Generally, the more expensive coolants offer the best protection, but make sure any coolant you buy meets AS/NZS 2108.1.97 Type A or B standards. Some vehicles - including a number of current Fords - call for a coolant purpose-developed by the manufacturer.
You should flush your radiator at least once every two years and, while you're at it, don't forget keep tabs on the condition of the drive belts, hoses, radiator cap and thermostat as well.
Brake Fluid
At present, the highest-grade brake fluid on the market in Australia is DOT 5. Like most other modern brake fluids, DOT 5 is synthetic, and has boiling point above 260 degrees Celsius. In comparison, earlier DOT 3 and 4 fluids boil at 205 and around 230 degrees respectively. Keep in mind that brakes get hot. Very hot. Brake fluid is the most critical fluid in your car.
A high boiling temperature is important in a performance application because there's less likelihood of the fluid turning from a liquid into a gas when it gets very hot. Once a fluid has vaporised, you get a spongy brake pedal or - in a worst-case scenario - a loss of braking performance.
Even for day-to-day duties, a DOT 5 fluid is preferable over DOT 3s and 4s. Brake fluid is hygroscopic, which means it absorbs moisture from the air. Absorbed over the course of a couple of years, this moisture reduces the fluid's boiling point and can lead to the aforementioned spongy pedal or a total loss of brakes. Starting off with a higher-grade fluid obviously allows lengthier use before there's a pressing need for replacement. The presence of moisture within the brake fluid can also lead to corrosion of internal brake system components.
Brake fluid should typically be changed every two years (depending on kilometres travelled and operating conditions) and it's also wise to check the condition of all brake lines - perished hoses are downright dangerous. They should be replaced as a matter of course every 10 years; more often in car that sees more - or harder - use. Note that many brake fluids are suitable for use in a hydraulic clutch application.
Grease
Grease is a soap mixed with lubricating oil, and different viscosities and temperature characteristics are required for certain applications. For example, wheel-bearing grease (subject to the intense heat of the brakes) must be capable of tolerating around 250 degrees Celsius, while most general-purpose greases cannot withstand these relatively high temperatures.
General-purpose grease is typically used for light duty applications such as suspension bushes, door catch and gear linkage mechanisms. Most general-purpose grease is water resistant, which allows its use on exposed body parts.
Probably the next most common grease in automotive circles is that used in CV joints. These greases are suited to extreme pressures and can be identified by their silver colouration. The silver comes from the inclusion of compounds such as graphite (carbon) that is an effective lubricant.
Usually, wheel-bearing grease is amber and general-purpose grease is brown. The consistency of grease is shown by its NGLI (National Grease Lubrication Institute) number, which ranges from 00 (for very thin) to 6 (for very thick). Most automotive grease is rated NGLI 2.
Note - never mix greases and, when repacking a wheel bearing, ensure that all of the original grease is removed from the bearing and casing.
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