Understanding Engine Oil

Getting your engine oil choice right has become increasingly difficult, leading Rob Marshall to investigate how it can be done, while unpicking the latest developments.

Getting your oil choice right has become increasingly difficult. We investigate more...

Even in today’s supposed ‘ecofriendly’ world, the link between the modern motor car and the petrochemical industry remains unbroken. From most interior plastics to the fuel that still powers over 98% of cars registered on our roads, perhaps the most obvious link to fossil-fuel is the engine oil.

Planes, trains and automobiles

Whether mineral oil, intended for a fourpot classic, or a sophisticated synthetic lubricant for a modern direct-injected turbocharged triple, the ‘base stock’ originates from crude oil. Yet, while nature provides the raw material, humans deliver the development. Petroleum-based mineral oil was less prone to solidifying when cold and leaving fatty deposits behind at higher temperatures, compared to animal and vegetable oils, making it more suited to the harsh environment of an internal combustion engine. Yet, the raw product remains from perfect.

One of the notable pioneers of lubricating oil in the UK was Charles Cheers Wakefield, a publicity-conscious entrepreneur, whose business diverted from railway lubricants to supplying engine oils for motor vehicles in the early twentieth century. While supplying buses initially, the Wakefield Motor Oil company became an engine oil sponsor for more exciting competitive events to prove its engineering prowess. This successful publicity campaign began not with motorcars but monoplanes, possibly looking to cash-in on the notoriety of Louis Bleriot, a French car accessory manufacturer, who flew to England over the English Channel in under 40 minutes. By adding a small proportion of castor oil to Wakefield’s mineral lubricant, the result was a superior oil film: the Castrol brand was born. Understandably, it was not long before attention moved to motorsport. Consequently, the 1911, 1912 and 1913 Isle of Man Senior TT races were all won by motorcycles using

Castrol oils. Development accelerated during hostilities, where the rangetopping Castrol R (‘Racing’) oil provided tactical advantages for fighter planes, due to the oil being able to retain its lubricating properties at the extreme low ambient temperatures that are typical of those experienced at high altitude.

During the inter-war period, Castrol R continued to be the engine lubricant of choice for British record breakers on land, water and air. However, passenger cars were becoming more popular but they had different demands to highlystrung race winners. Additives provided the answer. In the mid-1930s, Castrol’s research department discovered that adding tin and chromium elements helped to alleviate engine corrosion, internal sludging and oil ageing, caused by oxidisation. These issues were becoming noticeable on cars undergoing short trips and remain relevant today. Enhanced engine design and better quality fuels have played essential roles since but dispersant additives keep combustion deposits suspended in the lubricant. Ultimately, this saves the car owner from having to dismantle the engine and scrape away the carbon residues, in favour of a simple and relatively inexpensive oil change.

While racing lubricants provide oil brands with a degree of credibility, they were unsuited to everyday motoring. Excessive thickening at low temperatures makes the engine very difficult to turn-over and start from cold, especially for early motorists that had to wield a starting handle first thing in the morning. If the oil was specified to be thinner at lower temperatures, the compromise was excessive thinning when hot, courting engine damage.

Additives came to the rescue, once more. While lighter oils were introduced to the British motoring public by the 1940s, defined by their viscosity (‘thickness’), polymer-based viscosity index improver additives provided a solution to the lubricant thinning-out excessively at higher temperatures. Another additive, designed to suppress the pour-point and reduce the oil’s tendency to thicken at low temperatures, inhibits the oil’s wax molecules tendency to link together as they become colder. By adjusting the viscosity improvers and pour-point suppressant additives, therefore, engine oils could adopt different viscosity characteristics at set temperatures. This important invention laid the foundations for multigrade oils and the resultant Society of Automotive Engineers (SAE) classification remains in force today. The environmental role The internal combustion engine has always been extraordinarily wasteful and lubricants have become increasingly more important to reduce fuel consumption. Since the 1970s, engineers found that thinner (or less viscous) engine oils reduced resistance within the engine, wasting less energy. Engineers could use this to their advantage by improving performance, enhancing fuel consumption and reducing tailpipe emissions.

Naturally, motorsport provided an ideal testbed and BP’S introduction of a 10W-30 oil in its Formula Three racing car during the early 1970s raised eyebrows, when many motorists were used to using 20W-50 in their vehicles. The subsequent promotional material, to educate the public that engine oil choice can reduce fuel costs, started in the fuelcrisis era of the 1970s. Yet, legislation to limit carbon dioxide emissions especially has accelerated development. Five years ago, CM reported that Shell Lubricants had modified a Mitsubishi engine to run on an engine with an amazingly-low prototype 0W-12 viscosity oil, which has since become a production reality on some latest hybrids. In some instances, the oil is so thin that the engine internals are coated from new with special antifriction coatings to help tolerate potential extra metal-to-metal contact. Even so, 5W-30 is the most popular engine oil viscosity used by today’s car parc, supplanting the older 10W-40 grade.

As engines have become increasingly sophisticated, faster revving, hotter running and more powerful, mineral oil alone was no longer adequate. Synthetic oils maintain viscosity at high temperatures more effectively than mineral types, although (ironically) they are made from components that originate from crude oil. Unsurprisingly, modern chemical additive packs have also become increasingly complex. Yet, when catalytic converters became commonplace in the 1990s, which reduce toxic emissions by passing spent exhaust gases over a honeycombed structure of exotic catalysts, oil formulations changed again. As all engines burn oil to an extent, some of the anti-wear additives become deposited on the catalyst materials, rendering them ineffective. The problem was highlighted again in the 2000s, when diesel particulate filters (DPF) were blocking prematurely from anti-wear additives. This led to low sulphated ash, phosphorus and sulphur (SAPS) lubricants that must be used on engines that are equipped not only with DPFS but also Gasoline Particulate Filters (GPF).

The latest challenges

The current generation of petrol engines, especially, demands new generation oils. These small capacity, direct-injection, turbocharged units deliver impressive torque figures but require very low viscosity lubricants and extra additives to cope with the higher temperatures, increased deposits and extended service intervals. Unfortunately, certain calcium anti-wear additives, present within the oil, are responsible for igniting the fuel/air mixture within the cylinders prematurely. As the condition tends to be prevalent at low engine speed (<2,500rpm) but high-load conditions, LSPI (Low-speed Pre-ignition) is the latest challenge to overcome. Pre-ignition (‘pinking’) is just as damaging on these engines as earlier units that lacked advanced engine management that could retard the ignition automatically. Yet, because the ignition source is oil deposits on the cylinder wall, electronics cannot intervene. Violent explosions replace the intended smooth burning, the long-term consequence of which is severe mechanical harm, including damaged pistons and valves. While it is unclear whether the engine makers, or their lubrication partners, are to blame for this serious oversight, new oil standards are being introduced to help combat the challenge.

Apart from LSPI, the desire to reduce weight and carbon dioxide emissions has seen sump capacities reduce. While oil change intervals have not increased dramatically (if at all) within the last decade, many technicians and engineers concur that a biennial, or 20,000-miles, change interval is excessive. Various engine oil blenders have also revealed to CM that producing an engine oil that protects an engine throughout these extended drain periods is a significant challenge.

Identifying oil

The Society of Automotive Engineers (SAE) determines the viscosity measurement for mono and multigrade oils. Yet, the ‘weight’ scales for engine lubricant viscosities are not comparable to those used for EP (Extreme Pressure) lubricants, such as gear oils.

A lower number denotes thinner oil and an easier pour point. For multigrades, the ‘W’ denotes ‘winter’ and indicates the limit at which the oil ceases to flow properly. 0W will flow down to -30°C 5W up to -25°C

10W (-20°C)

15W (-15°C)

20W (-10°C)

Multigrades for classic cars tend to be mineral-based with minimal detergent additives. If your classic tends to work its oil to a very high temperature, or you drive frequently at motorway speeds, consider upgrading to an oil with a superior high-temperature capability.

We are all told that regular oil changes reduce motoring costs in the long run but making the wrong choice can be counterproductive.

While manufacturers have bespoke specifications that you should not ignore, this does not mean that you are limited to main dealer parts counters. There are many quality options but buy a recognised brand, or one that can justify its assertions.

PCV valves themselves can become clogged with sludge, resulting in the crankcase pressurising, which forces oil past the various rubber seals, causing oil leaks.

Oil vapour enters the inlet through the positive crankcase ventilation (PCV) valve. Check that the hose(s) between the valve and crankcase (including any fine gauze oil vapour separator) are unblocked.

This PCV valve is part of the cam cover, dictating that the entire moulding be replaced.

Image courtesy of Wynn’s Sludge deposits result, when the oil becomes contaminated. This can restrict oil galleries and increase mechanical wear. While flushing additives help, severe cases may require dismantling and mechanical cleaning.

Elderly oil seals harden and lose their sealing abilities. Quality engine oils contain additives to keep them supple. However, seal swell additives may help extend worn seal life, when dosed into the sump separately.

Image courtesy of Millers Oils Incorrect oil has such a negative influence on engine life that warranty companies tend to test samples of the engine oil before honouring (or rejecting) claims.

Modern engine oil pumps have their role to play with reducing power demands. The pictured BMW pump is of the volume-control design to ensure that no surplus quantities of oil are delivered under low-load operating conditions. Not only does this reduce fuel consumption but it also slows oil ageing.

Premature timing chain wear is becoming a noticeable problem on certain modern GDI engines especially and the latest oil specifications are evolving to provide extra protection to these parts.

Many modern cars, especially those fitted with turbochargers, possess oil-to-water heat exchangers, or oil coolers. Check them for leaks: this one is oozing both oil and coolant. Regular anti-freeze changes reduces internal corrosion, which risks engine oil mixing with the coolant.

Overfilling an engine sump can be just as damaging as allowing the level to fall too low. If your sump capacity is less than that of your oil filling can, do not add all the contents anyway. A clean measuring jug is a useful cheap purchase for this task.

The fine filtration channels on diesel particulate filters become blocked by certain anti-wear additives. They require low SAPS oil that complies with the ACEA C grades, at the very least.