Need viscosity clarification on differing Syntec products

[ToyotaNSaturn]

After being on this board for quite some time, I should know this but I'm confused now. After reading the following specs:

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Castrol Syntec: 5w-30 5w-50

Viscosity cSt 40C 53 110

LowTemp Cranking
Viscosity Index 171 175

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It seems that the 5w-50 meets the cold cranking spec to make it a 5w oil, but it MUCH thicker than it's 5w-30 counterpart at 40C. Wouldn't this wide variance hamper cold starting in the 5w-50 product? Wouldn't this variance also theoretically affect gas milegage? Are the Viscosity Index and Viscosity cSt @40C non-related test sequences?

[ June 15, 2004, 05:02 PM: Message edited by: ToyotaNSaturn ]

 

[Patman]

This is something that has puzzled me for a long time now. I believe that the 5w50 probably barely meets the 5w specs, and that if you ran this oil for an entire winter it would turn into a 10w before the halfway mark.

 

[Dr. T]

The problem is you are equating 40C with a "cold viscosity". There is nothing "cold" about 40C (~110F).

Yes, a 5W50 will pump like a 5W30 at -30C and will be thicker than a 5W30 at both 40C and 100C.

That's why the measurement at 40C is just a guide (a passing point in the scale as your engine is warming up). It is neither a "cold" engine temp. number nor really a "hot" engine temp.. The measurement at 100C is a better indicator of extreme heat viscosity.

 

[cweed]

I too usually look at the 100c viscosity when choosing oils....

 

[haley10]

I'm thinking that the viscosity index is an empirical number. It's only useful in comparing oils of the same viscosity grade. I don't place a high value on this spec. alone, because It depends on the chemistry.