The Engine Oil Bible
How much do you value the engine in your car? The life of your engine depends in no small part on the quality of the oil you put in it - oil is its lifeblood. People typically don't pay much attention to their oil - oil is oil, right? In the bad old days, maybe, but engine oil underwent something of a revolution in the 80's and 90's when hot hatches, 16-valve engines and turbos started to become popular. High compression engines and black death meant the days of one oil catering for everyone were over.
Take Castrol for example. They led the field for years with their GTX mineral oil. This was eventually surpassed by semi-synthetic and fully synthetic oils, including GTX2 and GTX3 Lightec. Those were surpassed by Formula SLX and most recently, Castrol GTX Magnatec. All manufacturers have a similar broad spectrum of oils now. I just mention Castrol in particular as they're my oil of choice.
What does my oil actually do?
Engine oil performs many functions. It stops all the metal surfaces in the engine from grinding together and tearing themselves apart from friction, and it transfers heat away from the combustion cycle.
Engine oil also holds in suspension all the nasty by-products of combustion like silica (silicon oxide) and acids. Finally, engine oil minimizes the exposure to oxygen and thus oxidation at higher temperatures. It does all of these things under tremendous heat and pressure.
How do I read the '5W40' type number?
As oils heat up, they generally get thinner. Single grade oils get too thin when hot for most modern engines which is where multi-grade oil comes in. The idea is simple - use science and physics to prevent the base oil from getting too thin when it gets hot. The number before the 'W' is the 'cold' viscosity rating of the oil, and the number after the 'W' is the 'hot' viscosity rating.
So a 5W40 oil is one that behaves like a 5-rated single grade oil when cold, but doesn't thin any more than a 40-rated single grade oil when hot. The lower the 'winter' number (hence the 'W'), the easier the engine will turn over when starting in cold climates. There's more detail on this later in the page under both viscosity, and SAE ratings.
A quick guide to the different grades of oil.
Fuel economy savings
Enhances engine performance and power Ensures engine is protected from wear and deposit build-up
Ensures good cold starting and quick circulation in freezing temperatures
Gets to moving parts of the engine quickly