A common misconception about motor oil is that if it looks dark in colour, it must be changed. This is not always necessarily true. Although it may be possible an oil service is overdue, the visual appearance alone is not a sufficient sign. There are a few simple methods you can use to assess whether an oil change is necessary.

1. Rub the oil between you fingers, does it feel gritty? If so, it could be an indication that the oil filter isn’t filtering out the oil well or the filter may be saturated. An oil and filter change would be necessary.

2. Smell the oil, does it smell burned? If so, the oil may have started to break down due to high engine temperature and mileage. An oil change would be necessary.

An oil analysis would be the best way to know for sure if your oil needs to be changed however this is not always a practical option.

When comparing two or more oils for possible use in your vehicle, the important thing to do is to compare technical specifications of each oil. Below we have listed not all, but some of the most important technical specifications needed to compare oils.

1. Viscosity Index (VI)

2. 
Cold Crank Simulator Viscosity

3. Pour Point

4. 
Flash Point

5. 

Total Base Number (TBN)

6. Sulphated Ash Content 


There are three types of synthetic oils, Hydrocracked, Polyalphaolefin (PAO) and Ester oils. Hydrocracked oils are not really synthetic at all, they are highly refined and modified mineral oils that due to a court case can legally be sold as synthetic oils. All cheaper oils and the 'synthetic' component of semi-synthetic oils are hydrocracked mineral oils. 

PAO synthetics are genuine, lab-made synthetic oils that are better lubricants than hydrocracked oils as they are built for their specific use, rather than the hydrocracked oils that are modified to perform a purpose. 

Ester based oils are the top end of oil technology and give the best protection available. The ester content (usually ester oils are mixed with PAO oils) has several functions that are very useful. Esters are electrostatically charged so they stick to metal surfaces, meaning that when the vehicle is started, there is already a layer of oil present. They are also more stable at higher temperatures, making them ideal as performance lubricants. The ester content also helps to make those oils better lubricants in general.

In general, yes. There are a few exceptions, Castor and plant based (as used in some biodegradable oils) are not safe to mix with conventional oils. Other than those few exceptions, mixing oil brands, types (synthetic, semi-synthetic and mineral) and viscosities is fine. The only problem with mixing oils is that the quality of the better oil is diluted by the lesser one.

There isn't really such a thing. If you look at the specifications listed on an oil, there should be an ACEA A and B specification, the A refers to petrol engine specifications and B to diesel. You will see that the numbers next to the letters are either the same or very close, meaning that the oil is suitable for both types of engine. 

In the case of cars with diesel particulate filters (DPFs/FAPs), they will often need an oil that meets an ACEA C specification, which relates to low ash oils. The use of oils that do not meet the correct ACEA C specification can result in the particulate filter becoming blocked, an expensive repair. 

Many car manufacturers have their own specifications and as long as the oil meets the relevant one, there is no need to consider the ACEA specifications as they will be part of the manufacturer ones.