Please don't merge this with another thread, as this is the only thread just to explain the basics of choosing the right engine oil to keep our beloved vehicles running forever (metaphorically)!
(If you guys like it maybe we should park it for other users' reference purposes) Disclaimer: I have NO endorsement with any company, neither do I have a personal interest/ulterior motive behind this article! Synthetic or Mineral?
Unfortunately, it is not that simple.
The basic difference is that mineral oils are extracted from crude oil stock by men in greasy overalls while synthetic oils are manufactured in labs by men in neat white coats!
But the way to actually judge an oil is not by it's origin, but by the seemingly boring aspects we typically overlook. Viscosity
Viscosity is the thickness of the oil.
This is always mentioned on the oil bottle, typically in the format "20W-50" or something similar for multi-grade oils. The number before the W (20 in this case) is the 'Winter' (Cold) viscosity, which is the thickness of the oil in a cold engine ie. at room temperature whereas the number after it (50 in this case) represents the 'Hot' viscosity at 98.9 Celsius ie. Engine operating temperature.
A higher viscosity number means the oil will be thicker. Viscosity of oil is rated at two temperatures because an oil is required to be thinner when the engine is cold, to allow easy starting, especially in very cold weather. But it must get thicker as the engine warms up to reduce wear and prevent component damage.
It is said that a lower viscosity will improve vehicle performance to some extent, as it provides less resistance to the movement of engine parts, and may even lead to a slight increase in fuel efficiency. Although this has never been properly proven, it has been reported, and I have experienced this before, although it was minor.
However, for long distance highway driving ie. above 130kph, you should never use a hot viscosity of below 30.
Synthetic oils were originally developed to allow lower viscosity oils that wouldn't freeze at extremely low temperatures. Oil additives
This refers to the non-lubricant chemicals in the oil.
These additives are the main component that set oils apart. They include corrosion inhibitors, viscosity modifiers (that allow oil to change viscosity with temp.), detergents and other compounds to help reduce friction and increase engine life. The main advantage of synthetics for the average user lies here, as Synthetic oils are compatible with a much better range of detergents and friction reducers, allowing us to replace the oil less often, increase engine life and economy, etc. Although manufacturer's precise list of additives are top-secret, they let us know the benefits of them. For example Castrol Magnatec contains magnetised particles that cling to the engine parts even when the engine is off and oil is not circulating and Castrol GTX (Mineral) has a heavy-duty detergent to prevent the buildup of sludge in the engine. Additives contribute to most of the cost of an oil, which is why good oils tend to be much more expensive, despite seeming identical on paper! Friction rating
This determines how good of a lubricant an oil is.
Contrary to popular belief, this is not interrelated with viscosity. It determines the amount of resistance an oil poses to movement when it is trapped between two moving components such as the Piston and Cylinder Wall.
It is rated by the American Petroleum Institute (API) on an alphabetical scale:
For petrol: SA onwards (S for Standard cycle), where a higher alphabet is better. ie. SN is better than SG.
For diesel: CA onwards (C for Compression cycle), where a higher alphabet is better. ie. CF is better than CB.
Thanks to modern technology, higher rated oils help reduce engine noise, vibration, harshness and wear. So prioritize this while choosing between oils of the desired viscosity. How often to change your oil
Changing too often can be as bad as changing too infrequently!
This is because the ability of some additives to function and make a difference actually increases with usage. So if you have invested in a good oil, do not be in a hurry to replace it, every 6000km for Mineral and 10000km for Synthetic is plenty for cars in Indian conditions! However, subtract 2000km from this is you do offroading or farm driving with a stock air filter.
Just make sure you change the oil filter everytime, or it will be a wasted exercise! Miscellaneous
Purely synthetic oils are only used in racing or -50Celsius applications, as they are infeasible otherwise.
Most oils advertised as synthetic are actually a blend of Synthetic and Mineral oils, to get a good balance of the advantages of both!
Make sure you check the size of the oil sump of your car (from the manual/internet) before you buy the oil! Now that you've confused us, how should we choose?
Well, it comes down to preferences.
If you like more responsiveness and typically stick to city roads, try a lower viscosity, but for India never go below 5W-30.
For highway driving at speeds above 130kph never go below 5W-40.
Feel free to experiment! Almost all of the oils in the market, whether synthetic or mineral, are compatible with one another. So you can change oils without needing an engine flush until you find the perfect one. If you are feeling very adventurous, you may even decide to mix (AT YOUR OWN RISK) two or more oils to get some advantages of both.
If your car is more than 10 years old, do not go below 10W-40, because the clearances with the engine would have become slightly larger due to wear and a thin oil may seep in some rare cases.
It is always great to check the recommended oil grade or friction level in your vehicle manual to know where to start your search, you could also contact an oil manufacturer such as Shell and ask if an oil you like would be suitable, as their staff is usually quite competent.
The normal oil that most service stations in India use is Mineral oil rated at API SG and 20W-40 viscosity because it is the cheapest option for them, especially in bulk. It's not actually too bad, but a good quality oil will easily make up for the price difference in the long term.
Changing the oil on a car is something that any amateur can do, but most service stations will not ask questions or charge extra if you tell them to put your own oil in during a service, you can always threaten them if they argue though.
Feel free to ask me questions or tell me if I missed anything!