Originally posted by revrev:
David, (MondeoST24)

Not quite. The first number (often with a "w" attached) i.e. 5w- or 10w- refers to the oils cold pour point. The oil's cold pour point is equal to a conventional single weight oil of the same number. This will affect how quickly the oil can be pumped through the engine. This would affect start-up lubrication at any temp, but especially at cold temps. It also will affect the parasitic drag of the oil pump at any temp.

The upper (-30 or -40) affects the shear strength of the oil, especially at higher temps. It does affect the pour point at all.

So, un summary: A 5-30 oil has the pour characteristics of 5 weight dino oil at all temps, and the shear characteristics of 30 weight dino oil at all temps.

Although the oil manufacturers guard their actual formulas carefully, I've been able to find some information that appears to be fairly reliable. Apparently, the Mobil 1 5-30 requires very few if any modifiers to have the pour point characteristics of a 5 weight oil and the shear strength of a 30 weight oil. The 5-50 requires more modifiers to enhance the shear strength up to 50. The problem with this is that the modifiers degrade faster than the base stock. So if you were measure a 5-50 oil when it was drained it may actually measure out at 5-40 or something like that. Also, the modifiers tend to only do one thing well, and displace the oil, so the more modifiers, the harder it is to get everything consistent lubrication and the more likely the oil will degrade. (Another reason NOT to use the snake oil additives. Ford specifically says DONā??T use them in the owners manual.)

Duratecs were designed for an oil with the flow characteristics of 5 weight oil. In very hot weather it probably wouldn't hurt it to have a heaver base stock, but there probably wouldn't be any advantage. And it was still in the car when there was cold snap, then there is the possibility of damage from the heavier oil.




Why did you edit -- remove -- the revealing performance data about dino and synth oils in your original post?