Originally Posted by lebikerboy
The first number (the "5" in 5w30) is only a relative number which basically indicates how easily it will
allow an engine to "turn over" at low temperatures. It is NOT a viscosity reference. In other words, a
10w30 is NOT a 10 weight oil in cold temperatures and a 30 weight oil in warm temperatures.
In fact, since SAE viscosity classifications only apply to an oil at 100 degrees C, it doesn't even make
sense to label it as a certain SAE viscosity at any temperature other than 100 degrees C.
Besides, if you thought about it for a second, it wouldn't make sense for a 10w30 oil to be a 10 weight
oil in the cold and a 30 weight oil in warm temperatures. What liquid do you know of that gets "thicker"
as its temperature increases or "thinner" as the temperature decreases?
You really need to do a little more research on oil technology and how oils are rated and by whom.
Multigrade oils are tested and certified to have a specific rating at two different temperatures - -18 centigrade for it's cold rating, and 100 degrees centigrade for it's hot rating.
The designation 5W-30 means that the oil was tested to flow as a 5 weight single grade oil at -18 degrees C. The "W" does stand for "Winter". It does not stand for weight.
That same multigrade was also certified to flow as a 30 weight single grade oil at the hot temperature. This does not mean that it is thicker when hot than it was when cold. It isn't. All oils thin as they are heated and that includes multigrade oils. A Viscosity Index Improver is added to standard base oils to prevent them from thinning as much as they would otherwise. This allows them to behave as a thin oil at cold temps and as a thicker oil would when heated to a higher temperature.
Synthetic oils behave a little differently and don't require VI improvers.
That's the simple explanation. I can't be bothered to go into any more detail here now - for something a little more technical see http://www.carbibles.com/viscosity.html
Here's another one - http://mysite.verizon.net/oldhokie/windyridge/oil.pdf
You might also want to investigate rating systems and specifications for different oils to see how they compare.
Also, if you still doubt what I've said then why not try posting what you did here at the bobistheoilguy forums and see how they respond? Those forums are populated by many tribologists and other petroleum experts and interested individuals. http://www.bobistheoilguy.com/
Here's a little bit of general information from that site. http://www.bobistheoilguy.com/oildefinitions.html