What oil might your engine like?

We receive a lot of questions about engine oil, e.g. what should be used, which brands are best, when should it be changed?

It's an absolutely vital ingredient for an engine, and although it looks very much as it always has done, and is as messy(!) as it always has been, it has evolved significantly over the years.

To most of us who enjoy driving, oil is the black runny stuff we only take any notice of when we find it dripping somewhere it shouldn’t. In truth, the technology behind modern lubricants is it’s a multi billion pound business that involves everything from chemists to engineers.

So what does oil do?  Well first and foremost it’s there to lubricate and reduce friction.  It also has 4 other functions:

So are all oils the same?  Definitely not!  Modern engine oils are no longer a basic product refined from crude oil.  There’s a multitude of additives they can and do contain, and oils can be mineral (traditional, old school oil) or synthetic, or a combination of the two, i.e. semi-synthetic.

Synthetic oils are manufactured using a more complex process than that used for refining mineral oils.  This allows control of their molecular structure and hence properties.

So how to tell oils apart?  Well oil specification is determined by two criteria, viscosity (or thickness) and performance (or quality).


High viscosity oils are thicker than low viscosity ones, and the thicker the oil is the slower it circulates.  Viscosity changes with temperature and the colder the oil the thicker it will be.

Go back far enough and engine oils were always single grade – so they tended to be too thick when the engine was cold and too thin when it was hot.  Definitely not ideal!  Fast forward then to the invention of multi-grade oils…

Multigrades contain additives called ‘viscosity improvers’.  These have the effect of making the oil thinner at low engine temperatures so ensuring quicker and better lubrication at start up (when most engine wear occurs), whilst allowing the oil to maintain its structure and lubricating properties (i.e. stay thick) at higher temperatures.

A multigrade oil has 2 numbers, e.g. 10W40.  The first number, in this case 10W (with the W standing for winter) gives the viscosity at low temperatures and the second number the viscosity at high temperatures.

The type and age of an engine will determine which viscosity oils should be used.

Modern engines are machined with very fine tolerances and run thin oils to give better emissions and fuel consumption.  Put an older, thicker oil in them and start up is harder, fuel consumption is worse, hydraulic followers will over-pressurise so valves won’t close properly, and the oil doesn’t quickly enough reach all areas on start up.

Conversely put a modern, thin oil in an old design of engine and you get problems as the greater clearances in an older engine require a thicker, higher viscosity oil to fill up the spaces and provide adequate lubrication and ensure metal to metal contact does not happen.


Turning now to the performance or quality of oils.  This has changed dramatically over the decades and the API or ACEA rating for an oil will give a measure of its quality.  Engine manufacturers will specify the rating required.  It’s OK to use a higher (invariably newer) performance oil in an older engine provided the viscosity is correct.

The semi-synthetic oils should have higher performance than mineral oils, and in turn fully synthetic oils will have even higher performance.

Although more expensive, synthetic oils have significant advantages over mineral oils.  These include: better flow rates and hence lubrication properties when cold; more resistance to breaking down at high temperatures; are thinner and so give better fuel consumption; don’t degrade so quickly.  In modern high performance engines, which put a lot of stress on oil, and may also have long intervals between oil changes, synthetic oils are usually recommended or indeed essential.

To conclude

The oil you choose for your car is critical. The wrong oil in the wrong engine can have serious effects.  We use a wide range of oils in our workshop – for example, an early Morgan +8 will have totally different requirements to a new Lotus Exige.  We ensure all oils we use exceed the engine manufacturers’ requirements, are recognised and respected brands, and so will give the best protection and life for your engine.

If you ever have any doubts as to what to put in your car, please do ask as we’re always happy to discuss and advise.