Let’s think about engine oil and its relationship to fuel economy in this issue. Using the correct engine oil keeps your engine running smoothly and efficiently, by stopping the metal surfaces in your engine from grinding together and wearing, primarily by creating a separating oil film between them. That means less friction, and less wasted power. Today’s engine oils also help prevent the build-up of undesirable deposits that can reduce fuel injector efficiency, protect against sludge and oxidation, and prevent corrosion.

Most car owners who don’t do their own DIY will rely on their garage to use the correct oil, but It’s getting more difficult to be absolutely sure that you do have the correct oil in your engine, as the choices, even within single viscosity grades like 5w 30, are becoming more and more complex. You may be sure that manufacturers want the very best for their engines, both in terms of life, and efficiency, so using the oil they specify makes sense, and is also essential to maintain warranty cover. For modern diesel engines with DPFs and catalyst boxes – most diesels made in the last ten years – the European standards authority ACEA ‘C’ series of oils covers such light duty diesel vehicles. Oils in this series, at present composed of oils graded C1, C2, C3, and C4, meet certain common standards, which are all very demanding. Within the “C” series there is a variation of up to 2.5 per cent in fuel economy, but also a variance in their ash, or SAPS (Sulphated ash, phosphorous, sulphur) level, with only C1 class oils combining both the optimum fuel economy with the lowest SAPS levels specified by some makers to avoid premature blockage of the diesel particulate filter.

The prospective fuel economy gains are relatively small, but significant, so that hasn’t stopped lubricant manufacturers from looking at every possible saving. Some manufacturers now specify lower viscosity 0w 30 grades to this end, rather than the normal 5w 30. So there are good reasons for the large number of different manufacturer specifications then, and you would be foolish not to adhere to them, but there are small garages that can’t keep the full range of oils covering all makes, and who may use something that’s a good quality ACEA C1, 2 or 3 series 5w 30, but which may not meet individual engine specifications, like VW 507.00, BMW Longlife4, or Renault RN 0720.

Some argue that more frequent oil changes are the key to good economy and engine life, and that this overrides meeting such precise specifications, and it’s possibly a valid argument on older cars without DPF systems. But, if you use a small independent garage for oil services, you should check out exactly what they are using in your car and then identify the correct grade from your handbook, or using on-line oil selectors. If their oil is not the correct one – it might involve an unacceptable SAPS/ash figure – then source the right oil yourself and take it along to the garage. Not only will it potentially save you some cash on oil charges, but it may help to avoid a congested diesel particulate filter, and eventually the sometimes huge cost of buying a replacement filter earlier than expected.

There have been some interesting developments from Millers Oils, the Yorkshire-based makers of the Diesel Power Ecomax fuel additive. As well as a wide range of conventional mineral, semi-synthetic, and fully synthetic engine oils, they offer a special range of energy-efficient engine and transmission oils using cutting-edge nanotechnology. This employs ultrafine friction-cheating spherical particulate additives called fullerenes, to improve fuel economy by lowering friction and prolonging engine life by reducing wear on key components. Independent tests have shown fuel savings of up to 2.5 per cent, and somewhat greater levels of power and torque gains, whilst these nanodrive oils have also been tested and proven on the racing track to maintain lubrication under extreme stress, when conventional oil films may break down. You will need Millers Oils’ ‘Which Oil’ selector on www.millersoils.co.uk to find the correct nanodrive grade for your engine. And if you really want to go the whole hog in pursuit of economy, this same technology is available in similarly race-proven gearbox oils, where transmission friction savings of up to 50 per cent can be obtained.

Victor Harman