Engine oil lubricates the moving parts in your engine, by forming a film between the different metal surfaces and keeping them from touching (and hence wearing each other) even under extreme pressures. That’s the lubricant’s main job, but oil has several other vitally important roles.

It prevents corrosion of the metals it coats, takes heat away from the combustion chambers (aiding the cooling system), and removes potentially damaging dirt particles from engine by transmitting them to the oil filter. Oil also reduces the friction between components – giving better fuel economy and power if the correct oil
is chosen.

There are three major types of engine oil. Mineral – a natural petroleum product refined from crude oil; synthetic – totally man-made and designed with specific properties for specific engines/conditions and Semi-Synthetic – a mix of the two that can be in varying proportions. Most modern cars will use at least semi-synthetic, with more and more designed to take fully-synthetic. Fully-synthetic is the obvious choice for most applications, especially in modified engines as it remains stable to much higher temperatures and pressures. The downside is that it is much more expensive. Also, some older engines and classics are still best run on mineral oil, as some of the chemicals in the synthetics can affect older gaskets and seals. Dedicated ’classic’ mineral-based oils have been developed with extra ingredients to keep aged engines healthy.

Look at any oilcan and it’ll have two figures displayed, for example 0w-40, 5w-40, 10w-50. These figures give the oil’s viscosity (a measure of the force required to break down the oil at certain temperatures and pressures, but, basically, how easily it flows). The first number (followed by a ‘w’ for winter) gives the cold viscosity and the higher figure is for when the engine is at full working temperature. Having the two operating grades is the reason such oils are known
as ‘multigrades.’

Now, it would be easy to assume that the best choice of multigrade would be the one with the highest differential (0w-60 for example), so that the cold oil gets to all the moving parts quickly on engine start-up and remains viscous and protecting well at high temps. Not entirely correct, however. 0w may be too thin to protect the engine fully during the initial warm-up stage and 60-grade is so thick, that even when at operating temperatures that friction is increased, resulting in higher fuel consumption and less power. Therefore choosing the correct oil grade is always
a compromise.

For a standard engine, it’s always best to go for the manufacturer’s recommendation. Ford will have spent much time and money figuring out what’s best for a certain engine design, so why go against that wisdom? The difficulty comes when an engine is modified for more power – usually with an increase in temps and pressures. Certainly, if you can afford it, a fully synthetic oil is best to use, and going for a slightly higher viscosity rating is a good move as the harsher operating conditions may mean a low-viscosity oil will start degrading with fast-road and track day conditions. Probably the best move for a well modified engine is to fit
a decent oil cooler. By keeping the temperature down to
more ’normal’ levels it will
take away the need to worry about changing grades from those that the factory recommends.