Michael / Jonmac :
I agree thoroughly with the comment that you shouldn't use lower viscosity
oil than an engine is designed for.
However, full synthetic has been available for some time in 20W-50, which
should be plenty thick enough for our old engines. I've in fact been
running my TR3 (rather hard at times) on it for several years with no
apparent ill effects.
BTW, the SAE weights are in fact directly related to viscosity measured at a
particular temperature, not to "lubricating capacity" or "shear and film
strengths". Lubrizol provides a nice chart at
http://www.lubrizol.com/referencelibrary/readyreference/6-OilClasses/physica
ls.htm .
Note also that "W" (as in 20W-50) does not refer to "weight" but in fact
"Winter", because the viscosity for the W grades is determined at a low
temperature (different for each weight). A particular oil formula is rated
by the lowest, then highest grade that it meets the standard for. So, for
instance, the '20W' part of 20W-50 means that it is no thicker than 4500
centipoise (cP) at
-10 C, the '50' part means it is no thinner than 3.7 cP at +150C.
AFAIK, there is no such thing as "30W", simply because the SAE has not
defined it.
Randall
> -----Original Message-----
> From: Michael D. Porter
> Both of these oils are full synthetics and have one characteristic in
> common. They are, quite simply, 5W oils. Most people assume
> that when an
> oil is rated 5W-30, it has the _viscosity_ of a 30W at operating
> temperatures and pressures. In fact, it has the lubricating
> capacity of
> a 30-weight oil, and there is a world of difference between that and
> viscosity. The latter figure indicates the approximate shear and film
> strengths of the lubricant when hot.
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