Oil is one of the most important components in a car, without it the engine would seize in minutes and the life expectancy of your engine depends upon the quality you use and how often you change it. The better the quality and the more often it is changed will mean your engine will last longer and run more efficiently. This however does not mean you have to go out and buy the most expensive oil you can because there are so many different types and some will not be suitable for your engine. An example of this is running a classic on pure synthetic (see below for definitions), this oil will not be suitable for all classics due to its make up and additives.
What does your oil do?
The main job of your engines oil is to lubricate between the engines metal moving parts and stop them from wearing each other away at the same time transferring some of the heat from the combustion. The engine oil must also be able to hold in suspension all the by products of combustion such as silicon dioxide and the acids produced as well. Additives in the all clean the engine to keep it free from the build ups and these acids while keeping the moving parts covered in oil.
Mineral Oil. Mineral oil is the stuff dug out of the ground and are then refined
Synthetic Oil. These oils actually come in 2 types and although there name is synthetic oils most are based on mineral (mobil 1) oils although they are refined from the purest part of the mineral oil. For this reason it is fine to mix mineral and synthetic oils in your engine although it is recommended that you remove one type before replacing it with another. The second type is a pure synthetic although these tend to be only used in the industrial sector for heavy loaded gearboxes.
Flushing Oils These are very thin oils so never drive your car with these oils in the engine as it will not last very long. These oils are designed to clean out the deposits in the oil ways of your engine. To use them, drain the oil engine oil but leave the old filter in place. Fill up with flushing oil and then run the engine at fast idle (very low load) for 20 to 30 minutes then drain the oil and see what comes out.
Change the oil and filter and then fill up with your preferred engine oil. Be warned using flushing oil can sometimes do more harm than good especially in an old engine ( in most classics) as the flushing oil removes deposits which have built up over the years, these deposits can seal up parts of the engine (sump gaskets, head gaskets and even seal the piston rings). Once the deposits have gone the engine can leak or worse blow past the pistons rendering the engine scrap.
Different grades of oil
The following table is a quick reference guide to the different grades of oil. The oils get "better" the higher up the table but as said before the best oil might not be suitable for your engine so it is important to consult the manufacturers handbook
Much improved fuel economy
Enhances engine performance and power
Ensures engine is protected from wear and deposits
Ensures very good cold starting, quick circulation in low temps
Gets to the moving parts of the engine quickly
Good protection within the first 10 minutes of starting
Approximately 3 times the protection at reducing engine wear
Increased oil change interval but less than for mineral
Basic protection for most engines (classics)
Oil needs to be changed more often
So what do the numbers mean?
The numbers refer to the viscosity of the oil which is the most important criteria for lubricating oil. The thicker the oil then the higher the viscosity it will have. Because we operate engines over different temperatures multi grade oils were invented, these work by having a polymer added which stops the oil thinning too much as it gets hot. So what do the numbers mean? at low temperatures the polymers are coiled up and allow the oil to flow as its low number ie 15W. As it heats up the polymers unwind to from long chains which prevent the oil thinning too much. This means at 100C the oil will only thin to its higher number ie 40. In this example it is a 15 weight oil which will not thin more than a 40 weight oil when it gets hot.
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