>Date:    Tue, 6 Feb 2007 14:53:31 +0000
>From:    Dennis Haynes <dhaynes@OPTONLINE.NET>
>Subject: Re: Castrol, QT, 5W50
>
>Using conventional oils, 20w-50 is the preferred oil for normal temps and operation. For low temaparatures, 20F is really the comfortable lower limit for this viscosity. Synthetic lubes can be extended for both extremes as not only do they have better viscosity indexes, they do not have the waxes that will solidify at lower temperatures. The viscosity ratings work as follows. The lower number is the viscosity at 40C. The upper number is the viscosity at 100C. There can be major differences at temperatures far above or below this. This is the differences we do not see in the ratings.
>
Actually, neither number is the viscosity, but rather relates to a
particular viscosity *range* (SAE 30W for e.g. is 9.3-12.5 cSt @100C).
The test temperature for the lower viscosity number (i.e. 10 for 10W-30)
is 30C below that number. I.e. -20C for 10W30, -10C for 20W50, etc.  So,
20W50 will only meet the SAE "W", or Winter, classification for
temperatures above -10C  (14F).

Keith Hughes
'86 Westy Tiico (Marvin)