FAQs

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What is oxidation?

Oxidation is the chemical reaction with oxygen, which permanently changes the molecular structure and therefore the properties of the oil. Depending on the extent of oxidation, the oil molecules become replaced by other chemicals such as alcohols, aldehydes, ketones and acids which lead to an increase in oil viscosity, as well as deposit formation, varnishing and sludge.

What’s the best way to stop oxidation?

The bad news is that oxidation of hydraulic oil is unavoidable. The good news is that it can be slowed and managed by selecting the right oil and implementing oil maintenance regimes. The rate of oxidation can be accelerated by factors such as high temperature, and the presence of acids, water and wear metals e.g. copper, or slowed by the use of anti-oxidants and inherently stable base oils. By taking regular oil samples, the presence of any contaminants can be identified and remedial action such as filtration can be taken.

What impact does oxidation have on hydraulic systems?

Oxidation reactions reduce the amount of functional oil in the system and replace it with chemicals such as acids which promote corrosion and loss in foam control, and cause an increase in viscosity, as well as deposit, varnish and sludge formation. In turn, these restrict oil flow by clogging filters, strainers and valves. Varnish build up can act as an insulating film, increasing operating temperatures and accelerating the rate of oxidation, while deposits cause wear in the system. The ultimate result of oxidation is system breakdown – most often caused by pump failure.

How can you tell if an oil has oxidised?

Two perceptible indications of oil oxidation include foul odours and darkening of colour, as well as the more obvious presence of sludge and varnish. There are also a number of ways to determine the level of oxidation by sample analysis in the lab:

Total Acid Number (TAN): Fresh hydraulic oil should have a TAN of around 0.2 mg KOH/g, however, this will increase as acids are produced in oxidation reactions. Once the oil has a value of 1 mg KOH/g greater than the initial value, an oil change is strongly recommended.

Viscosity: Oxidation causes an increase in oil viscosity, as the acid molecules produced in the reactions associate, increasing the median density and therefore viscosity.

IR spectroscopy: Oxidation products such as alcohols, aldehydes, ketones and acids can be identified in the oil by using infrared spectroscopy. A greater presence of these molecules indicates a higher level of oxidation.

Millers Oils offers a sampling service to keep maintenance departments informed of the condition of their oil and equipment, so maintenance can be planned around operations, saving both time and money.

Why do different gear boxes require different oils?

Different types of gears are used for different applications, and selection depends on several variables including gear ratio, load, and noise tolerance. The key types of industrial gears include spur, helical, bevel and worm, and in each type the gear teeth mesh in a different way, meaning the metal-to-metal contact and therefore lubrication requirements vary:

How do you select the right gear oil?

Gear oil should match the recommendations of the original equipment manufacturer (OEM), which will either be stated on the gear box itself or found in the manual. Often there will be several products that could be suitable for your application. Millers Oils Tech Help Desk can advise the best product to match both the OEM recommendation and your particular application.

Key properties of gear oil include:

  • Viscosity: gear oil must be thick enough to maintain lubricating film, but not to cause drag and energy wastage.
  • Oxidation stability: anti-oxidant additives boost oxidation resistance so the oil lasts longer – this is particularly important at higher operating temperatures.
  • Thermal stability: synthetic oils offer superior thermal stability so the oil lasts longer, especially at higher temperatures.
  • Extreme pressure performance: high loads result in high pressures that squeeze out the oil between the gear teeth – EP additives prevent wear and damage in these conditions.
  • Demulsibility: gear oil should separate quickly from water (with the exception of PAG based oil).

What is micropitting?

Micropitting, also called grey staining, is a type of wear caused by metal-to-metal contact at microscopic level and leaves the metal surfaces with a ‘dull’ appearance. Although the surfaces of gear teeth may look smooth to the naked eye, microscopic inspection reveals roughness and asperities that break through the oil film – especially at high temperature and high load, and high speed and low torque. Micropitting can be avoided by selecting the correct viscosity of oil (i.e. high enough to provide a good film thickness), and by using oil with extreme pressure additives.

What are the benefits of synthetic gear oil?

Although modern gear oil additive technology is extremely advanced, using base oils that offer inherently better properties can reduce the additive level required, and/or further improve the performance of the gear oil.

In general, synthetic oils offer:

  • Better lubricity for better gear teeth protection and reduced wear, extending the life of the gears.
  • Improved viscosity index, meaning better and more consistent performance across a wider range of operating temperatures.
  • Higher thermal and oxidative resistance, so the oil is less likely to degrade due to high temperature or contamination, meaning fewer deposits and less sludge, as well as extended drain intervals.
  • Reduced volatility, evaporation and flammability, making them safer to work with.

What’s the difference between mineral, PAO and PAG based gear oils?

Mineral oil is the most commonly used base oil in gear oil. It offers good lubricity and corrosion protection to protect gear teeth against wear, and is often formulated with EP additives for extra protection at high loads.
The most common synthetic base oils used in gear oil are Polyalpha olefin (PAO) and Polyalkylene glycol (PAG). Both PAO and PAG have high viscosity indexes, meaning better and more consistent performance across a wide range of operating temperatures compared with mineral oil. They also have better thermal and oxidative resistance, so the oil is less likely to degrade due to high temperature or contamination, and will offer extended drain intervals relative to mineral oil.

PAG oils are often recommended for worm gear applications, as they are very effective at
dispersing the heat generated by the high speeds and friction created when the worm gear teeth mesh.

It’s important to note that PAG oils are not compatible with mineral or PAO oil. If you are changing from one type of product to another, you must do a full clean out and flush of the equipment.

Millers Oils offers three different ranges of industrial gear oils that use different base oils:

What are EP additives?

EP stands for extreme pressure. EP additives are used in oil formulations where there are high pressures involved (e.g. gearboxes) that can cause the oil to be pushed out from between the two metal surfaces, risking metal to metal contact and wear. Once the oil film begins to fails, the EP additives kick in, reacting chemically with the metal surfaces to form a sacrificial surface film that prevents welding and seizure and protects the components.

What is ACEA?

The European Automobile Manufacturers’ Association (ACEA) is the main lobbying and standards group of the automobile industry in the European Union. Among many other activities ACEA defines specifications for engine oils so called ACEA Oil Sequences. The sequences are usually updated every few years to include the latest developments in engine and lubricant technology. ACEA itself does not approve the oils, they set the standards and oil manufacturers may make performance claims for their products if those satisfy the relevant requirements.

Will non-approved oil affect my warranty?

Yes using a non-approved oil can lead to a possible void of your warranty. If an oil is approved to meet the required specifications then it cannot be held accountable for damaging your vehicle because it has been approved. Therefore, using a non-approved oil which might fit all the specifications of the vehicle manufacturer but has not been approved can void your warranty because there is no way of proving it hasn’t led to the damage of your vehicle without this approval.

What is an OEM approval?

Many oils state approvals for various brands of vehicles or original equipment manufacturers (OEM’s). Oil companies send their oils to OEM’s for approval. An OEM approved oil is one that has been developed to the specification of an OEM using a blend of base oils and additives that suit the engines requirements and is then extensively tested to ensure full compliance. The extensive testing and subsequent approval is the crucial difference between meeting specification and OEM approval.

Does it matter which oil I use in my car?

Yes, using the incorrect oil can cause greater wear on the gears and bearings, which means higher maintenance costs and, in time, could lead to engine failures.

What is viscosity?

Viscosity is the “thickness” of a fluid, and is an important characteristic of a lubricant. The viscosity determines the film thickness and film strength of an oil, which are critical for reducing friction and keeping metal surfaces apart.

It’s important to select the right viscosity oil for your vehicle or application – check the OEM specification or use our Which Oil? selector.

High viscosity or “thicker” oil is not necessarily better, as if the viscosity is too high it can cause excessive energy consumption, heat generation and poor start-up lubrication.
If a viscosity is too low, there will be oil film failure and insufficient lubrication, which then leads to friction and wear as well as sensitivity to particle contamination.

Do I need different engine oil for diesel and petrol engines?

Generally the answer is yes for older vehicles, as diesel engine oils have higher detergency levels. It is important to refer to your vehicle manufacturer’s handbook to establish exactly which specification of oil is required. This is vital if your car has extended service intervals and you are still within the warranty period. New vehicles are designed in a way that allows the same oil to be used whether the engine runs on petrol or diesel.

Why is synthetic oil more expensive than mineral oil?

A synthetic oil is produced in a more sophisticated and expensive manufacturing process. A fully synthetic oil does have many performance advantages which justify the additional cost. Fully synthetic oil allows longer service intervals, reduces oil consumption and contributes to improved fuel economy – all of which reduce overall running costs.

What is multi-grade oil?

A multi-grade oil refers to the viscosity of the oil. Viscosity is the ‘thickness’ of a fluid, which is an important property for lubricants as it helps determine how well the oil flows around the components, as well as how strong the oil film is. In general, the viscosity of an oil will decrease with increasing temperature.

Multi-grade oils contain additives to ensure they maintain their viscosity better over a wider temperature range, to ensure that components are protected at low start-up temperatures as well as high running temperatures.

Multi-grade SAE viscosity classifications consist of two numbers, e.g. 10w40.
The first number is the low temperature specification (the ‘w’ stands for winter), or the ability of the fluid to flow when cold (pumpability when cold cranking), even at temperatures as low as -35˚C.

The second is the high temperature specification at 100˚C, to replicate when the engine is running at operating temperature.

What can I do to counter the effects of ethanol in modern fuel?

Ethanol is a renewable fuel, and modern petrol usually contains 5% ethanol. Today’s vehicles are designed to run on this fuel, but in older vehicles the ethanol content can cause corrosion.

Millers Oils offers a range of fuel treatments to protect classic cars from the adverse effects of ethanol, meaning they can still use the fuels available at today’s pumps.
The Federation of British Historic Vehicle Clubs (FHBVC) conducted tests on our VSPe Power Plus, VSPe and EPS fuel treatments and in April 2012 endorsed them as offering protection against the corrosion effects of ethanol.

Can I use synthetic oil in my classic car?

Yes, after researching into synthetic technologies and additives, Millers Oils is able to provide products for classic vehicle that not only eradicate historic issues with synthetic products, but further protect your vehicle and have the potential to enhance performance – see our blog Dispelling the myths around synthetic oil in classic vehicles for more information.

What is ZDDP?

Zinc Dithiophosphate (ZDDP) is an anti-wear additive used in lubricants. ZDDP is required to protect high load contact points such as camshaft lobes and followers. It also acts as an antioxidant to extend oil life under harsh conditions.

There is an optimum level of ZDDP: too little won’t offer enough anti-wear protection, but too much actually increases friction and wear.

The ideal level is 1000 – 1400 ppm, and all Millers Oils engine oils use this optimised amount.

It is important to note that a product with 1400 ppm ZDDP is not necessarily better, as the level has to be balanced with other additives in the formulation to ensure all-round performance.

How does race type affect oil selection?

A key consideration for motorsport oil selection is the type of race in which a vehicle is competing.

Certain race types, e.g. Rallying, place the engine under high stress, increasing operating temperatures and pressures. With these race conditions it is often recommended to increase the viscosity of the engine oil.

Some races, for example endurance racing, require improved fuel economy which can be achieved by reducing the engine oil viscosity. By switching from a 5w40 to a 5w30 grade, fuel consumption can be reduced by up to 3%! Lower viscosities are also favoured when there is a short or no warm-up time, as the oil flows more efficiently on start-up.

How do vehicle modifications affect oil selection?

Modifications to vehicles can effectively make the original oil specification and viscosity redundant.

As power is increased, the internal temperatures and pressure of the engine rise, meaning the oil has to work harder. In general, the more stress placed on the oil, the more the required viscosity will deviate from the original vehicle specification, in order to maximise film strength and protection.

Does motorsport oil meet passenger car specifications?

Millers Oils motorsport products do not claim automotive specifications, as the performance requirements are different to those for road vehicles. Motorsport oils are superior at higher temperatures and pressures, but they are not always formulated with the level of detergents and dispersants needed for today’s heavily regulated passenger car market.

How often should I change my oil?

Millers Oils does not recommend oil change frequency for vehicles – this is determined by the vehicle manufacturer’s specification, usually based on distance or time. However, Millers Oils does recommend oil sample analysis, as this can pick up on any signs of premature engine wear or contamination that could indicate oil may need to be changed sooner.

What is a DPF?

DPF stands for Diesel Particulate Filter, and it is a post-combustion device fitted within the exhaust system. The DPF physically filters soot and other particulates to avoid release into the environment, thereby reducing emissions.