Eric,
I'd ad that the base oil for multigrades is actually less then the "W"
oil; e.g. for 20W-50 the base
oil is about 15, since the VIs (viscosity improvers) have an effect when
the oil is cold.
The main problem with straight-weight oils is that they're so thick when
cold--you run the risk
of starving the top end of oil for a few seconds after start-up. OTOH,
the straight-weights MAY
adhere to the surfaces better, offering some protection till the stuff
gets flowing.
There's a LOT of debate about this in the aviation community. The
consensus is that if want the
best of both worlds, or live in a climate of extremes, run the
straight-weight in the summer and the
multi-grade--or at least a lighter straight-weight--in the winter.
bs
Frenken, Eric wrote:
> Alan,
>
> I know for sure that on multigrade oils like Castrol GTX 20W50 the polymers
> (kind of molecular chains), which are responsible for the attribute
> 'multigrade', deteriorate with time, turning the multigrade oil into a single
> grade oil with a viscosity of the basis oil of the multigrade oil (in your
> case 20). This process is sped up when the oil is used, resulting in lower oil
> pressure (mainly noticeable when the engine is idling), because an engine oil
> with a viscosity of 20 is too thin for our cars when driving in summerly
> temperatures.
>
> That said multigrade oil will deteriorate even when it sits on the shelf,
> although this process is slower than when it is in use. That's one of the
> reasons I use single-graded oil in my engine with enough ZDDP and only little
> detergent and little dispersion additives in it. That oil was used when our
> engines were designed and the manufacturers had to design the engines with the
> attributes of this oil in their mind.
>
> I have a more constant oil pressure since using this oil. Even oil consumption
> (caused by burning oil and leaking) and engine temperature dropped down a
> bit.
>
> Switching to synthetic is no solution, but YMMV.
>
> Eric
> Heinsberg/Germany
> 123ignition dealership
>
>
>
>
***************************************************************
Bob Spidell San Jose, CA bspidell@comcast.net
'67 Austin-Healey 3000 '56 Austin-Healey 100M
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