Brendan,

In an effort to get this thread up to my forecast 50 hits...and to explain what I think I know about multi-grades here is some dope!

If you take a quart of XXX oil and drop its temperature to zero, then measure its viscosity with the 'special viscometer' made for 0&deg oil it might read as a heavy old 40 weight. Mr. Chemist starts adding pour point depressants in until the oil flows through the 'special viscometer' and reads as a 10 weight! Mr. Chemist is halfway done...the oil flows as a 10W (W=winter which means measured at 0&degF), which means it has these characteristics as 0&deg F.

Now the oil his heated to 210&deg F and measured on a different 'special viscometer'...one calibrated for this temperature. The oil flows too easily, almost like water. Mr. Chemist adds viscosity improver's until the oil flows through the 'special viscometer' with the characteristics of a 30 weight oil. Now his job is done. The oil flows as a 10 weight at 0&deg and flows as a 30 weight at 210&deg...he has created a 10W-30 motor oil.

The original XXX oil would be very heavy at 0&deg and then the heat would thin it considerably at 210&deg. The modified oil doesn't do this and Mr. Chemist gets the Nobel Prize for his invention.

So...this oil doesn't get thicker with a rise in temperature, it just doesn't get as thin as a virgin oil would because of the goodies in it.

Stuart