Auto Parts: How about the motor oil recommended?, viscosity index, long chains

Question
I have a 2000 passat 2.8L ATQ wagon car.
Which kind of engine oil and brand is the best recommended?
I just noticed that after I refilled the castrol 5w-40 oil,
the milleages pergallon change from 24 to 18 mileages/pergallon?  What is the reason?
Thank you very much.

Answer
Hi Peter,

Motor oil recommendations. You won't come accross too many good technicians that won't recommend Valvoline or Castrol.

The weight of the oil you want to get depends on where you live and what engine your vehicle has. I know what engine I'm working with but not sure where you are so...
...I'll just give you a quick bit of info.
5W-40 motor oil is multi-viscosity. That means that the base lubricant (5) is the lowest viscosity your oil will achieve during it's usable life (viscosity breaks down exponentially beyond the usable life). The 'W' indicates that the oil was tested and meets specifications for viscosity a 0 degrees Fahrenheit and is suitable for winter use. The final number indicates the 'real' viscosity of the oil as tested at selected temperatures.  

Viscosity Index is an empirical number indicating the rate of change in viscosity of an oil within a given temperature range. Higher numbers indicate a low change, lower numbers indicate a relatively large change. The higher the number the better. This is one major property of an oil that keeps your bearings happy. These numbers can only be compared within a viscosity range. It is not an indication of how well the oil resists thermal breakdown.

Multi viscosity oils work like this: Polymers are added to a light base (5W, 10W, 20W), which will prevent the oil from thinning too much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 220 degrees F the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-viscosity oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.

Ok, why did your fuel mileage drop? Well, your engine also uses horsepower it creates to pressurize and distribute oil throughout the engine. A thicker oil will require more power to compress and move which will result in less fuel economy. This shouldn't be understood that the lighter an oil the better fuel mileage one will achieve, this IS true but your engine determines the LOWEST viscosity it can use. Your manufacturer recommends 5W-30 to be used but as your engine ages and crests the 100K miles mark, you should increase the first number by one step. In your case it would be 10W-30. Also, Castrol uses synthetic as well as conventional detergents. These additives cause the viscosity to stabalize early in the heat-cycle which keeps the oil 'thicker' longer which will also reduce fuel efficiency. 10W-30 Valvoline will probably be your engines best friend because it has high lubrication abilities, reaches it's heat cycle very quickly and maintains viscosity without high amounts of synthetic polymers or detergents.

Thanks for the question, I hope this information is helpful to you.
Kb