Viscosity question

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May 24, 2003
Baltimore area
Still consider myself fairly new here. This has become my favorite forum to read. Anyway, enough butt kissing. I'm looking at the spec sheets for Castrol GTX and Syntec Blend and comparing them. In particular I'm looking at Viscosity, cSt 100C and Viscosity, cSt 40C numbers. I'm assuming that a larger number is better. Am I correct? For GTX 10W-30, the cSt 100 and cSt 40 are 10.7 and 72 respectively. For Syntec Blend in the same weight, the same two specs are 11.5 and 79. Does this mean that the Blend (having higher numbers) stays in its weight better, or does it mean that the oil is on the higher end of its weight? Or does it mean both, or neither? I admit I don't understand everything that I read on this forum.
It just means that it is at the higher end of the weight. 40C is an arbitrary temperature used only to calculate viscosity index. If you look at the Cold Cranking viscosity you would probably find the blend is thinner when cold.
It just means the 'blend' is ever so slightly thicker at those 2 temps...and that's can't extrapolate whether it's better nor worse...a higher number at 100C should mean an ever so slightly thicker film thickness if so desireable, for added protection.
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