Use the exact oil grade and required approval your engine was designed for. The number on the bottle matters, but the required quality rating matters too.

The first number shows how the oil flows when the engine is cold. A lower number means it moves more easily at startup, especially in cold weather. The second number shows how thick the oil stays after the engine gets hot. A higher number means the oil stays thicker at normal running temperature. This does not mean thicker is always better, so the best choice is the one your engine was designed to use.

Even if two oils have the same viscosity, they can still be very different. Each car manufacturer sets certain specifications for certain engines, and there is no one oil that fits all engines. Two oils may both be, for example, 5W-30, but one may meet the required specification for your car while the other does not. That is why matching the viscosity alone is not enough — the oil must also match the required specification for the engine.

Yes. Some manufacturers require more than just the correct viscosity and API level. They may also require a specific manufacturer approval or industry standard for certain engines. That is why two oils can have the same viscosity but still not both be suitable for the same car.

If needed, yes, but only as a temporary solution. It is always better to use the same oil grade and specification. Mixing different oils should not become a habit.

Using the wrong oil once does not always cause immediate damage, but it is still not recommended. The oil should be replaced with the correct one as soon as possible.

Not always. An older car should still start with the viscosity recommended by the manufacturer. A thicker oil is only sometimes considered if the engine is already using oil between changes.

Yes. Even if the car is not driven a lot, the oil still ages over time and should be changed according to the recommended time interval.

No. Topping up only restores the oil level. It does not replace the need for a full oil change.

That is usually a good sign, not a bad one. Engine oil contains detergents and dispersants that help clean the engine and keep dirt, deposits, and combustion by-products suspended in the oil. As it does that job, the oil often becomes darker. Oil that stays unusually clean for too long is not automatically a good sign. Color alone does not tell you whether the oil is still good or needs changing.

The main way is to follow the oil change interval in the owner’s manual or the car’s oil life monitor. That is the most reliable guide, because oil can wear out even when it still looks normal.

Other signs it may need attention sooner are:

• the oil change reminder comes on,
  
• the oil level drops too low,
  
• the engine becomes noisier than usual,
  
• you notice a burning smell, or
  
• the oil has been in the engine too long by time, even if the car was not driven much.
  

Also, color alone is not a reliable way to judge oil. Oil can turn dark because it is cleaning the engine, so dark oil does not automatically mean bad oil.

1. External oil leak

First, check if the engine is leaking oil from the outside. A good way to do this is to clean the engine first, then inspect it again after some driving. If fresh oil appears, it can help show where the leak is coming from. Also check under the car for oil spots.

2. Internal oil consumption

If there is no visible leak, the engine may be using oil internally. One common sign is blue smoke from the exhaust, especially when revving.

If the oil level keeps dropping, the engine should be checked.

Do not leave the oil above the MAX mark. Too much oil can cause engine damage, so bring it back to the correct level.

Too little oil puts extra stress on the engine and can cause damage. Top it up before driving far.

Park on level ground, stop the engine, wait a few minutes for the oil to drain back, wipe the dipstick, put it fully back in, then pull it out and read it. The oil should sit between the low and full marks.

Transmission fluid should be changed at the interval listed in the owner’s manual. Some cars need it sooner under harder driving conditions, and different transmissions can have very different service intervals.

Because they are talking about different service methods. A simple drain-and-fill only removes the fluid that can drain out, so some old fluid stays inside. A full exchange replaces much more of the old fluid.

There is more than one way to change transmission fluid, and that is why the amount of old fluid removed can be different.

Drain-and-fill

This is the simplest method. The old fluid is drained from the transmission and new fluid is added. It only removes the fluid that can come out by gravity, so some old fluid usually stays inside.

Pan removal service

In this method, the transmission pan is removed. This allows the fluid in the pan to be drained, and it may also allow access to the filter if the transmission has a serviceable one. It also gives a chance to inspect the pan for dirt or metal particles.

Full fluid exchange

This method replaces much more of the old fluid, including fluid that stays inside other parts of the transmission. That is why it usually changes a larger amount than a normal drain-and-fill.

So in simple terms, the method used affects how much old fluid comes out, whether the filter can be changed, and how complete the service is.

Sometimes yes, sometimes no. Some transmissions have a serviceable filter that should be changed during service, while others do not have a filter that is normally replaced. It depends on the transmission design.

Yes. Transmission fluid must match the exact type and specification required for the transmission. Using the wrong fluid can cause poor shifting or damage.

No. CVT fluid and normal automatic transmission fluid are made for different transmission designs and should not be used interchangeably unless the fluid is clearly approved for that exact transmission. Using the wrong one can cause poor performance or damage.

No. DCT fluid and normal automatic transmission fluid are also different and should not be used interchangeably. Each transmission type needs the correct fluid and the correct specification.

No. These are different transmission types with different fluid requirements, so the fluid must match the exact transmission design.

No. There is no single transmission fluid that safely fits every transmission.

No. “Multi-vehicle” does not mean universal. It is only suitable if the required specification for your transmission is actually listed.