MH>So the oil is actually thicker at hot temperatures or when it is hot outside
MH>you are using a 10w 30?  10w for cold and 30 for hot?

That is corect. A 10W-30 is the same as a 10 weight when it is cold but
the oil changes as it warms up to the equilivent of a 30 weight when
hot.
Here in SAn Diego California where the weather is fairly mild
I had been running 20-50 oil in my cars.

I have recently changed to using a full symthetic and will be using the
10-40 in my older truck with a little over 100,000 miles on it.

Becasue of the increased gas miledge helps then meet the CAFE standard
most car manfacturers recommend the 5-30 or 0-20 in their new cars
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 þ SLMR 2.1a þ Typo Tom strikes agaoin