The multigrade oils arent so multigrade anymore after about 6000 - 8000 miles (we tested that in a Lab). You can try this at home. Poure a cup of 6000 mile oil in to a smal funnel and stop the time it takes till it is empty.
Than try this with the same brand@same temperature with the new oil.
Do it with oil out of a refigator and with Oil heatet to 100 deg Celsius and you will see what I mean.
Same is with the addditives in the high price mineral and synthitic oils.
They break down pretty soon.
I found this on te Wearchek website:
If a multigrade oil's viscosity stability with varying temperature is so useful, what is the point of having monograde oils? The reason is that a number of the additives used to enhance the VI of an oil are unstable in a working environment. The biggest problem is that they tend to shear, that is, physically degrade....
If a multigrade oil is subject to high shearing stress the additive which imparts the high VI will start to break up, resulting in a sharp drop in viscosity. This could be detrimental to the component, causing the oil to lose its load bearing characteristic and, under these circumstances, a monograde oil might be the safer choice.
I recomend Shell Rotella SAE 30 because it will not ruin your cam like the newer Oils for modern cars plus a cup of your favourite "snake oil" additive.
We use Mathy here in Germany for 50 Jears with succsess, but I don`t think they sell it in the US.
AV-Blend has a similar formula an is a US Product.
http://www.avblend.com/ Frank
