> Changed from Castrol Syntec 5w50 to 10w30 a few days ago - (I figured the
> 5w50 might be a little thick
Multi-viscosity ratings can be a little misleading. Since multi-vis oils
are created by dumping polymers into a base stock of varying viscosity (5W,
10W, 20W), the 10w30 is actually more viscous (i.e. "thicker") than the 5w50
at the SAE test temperature.
As the oil temperature rises, the polymers will uncoil and prevent the
oil from thinning out as much. At 100degC the oil will have thinned as
much as the upper viscosity rating indicates. So your 5w50 is a 5W oil
that will not thin more than a 50 weight oil would when hot, and likewise
for the 10w30.
Personally, I agree with your switch, but mainly because of the wide
viscosity index range. There are a lot more polymer additives in a 5w50 than
a comparable 10w30. Many synthetic oils can claim multi-viscosity ratings
with no viscosity index improver additives at all (e.g. AMSoil 10w30 has no
VI improvers, but AMSoil does add some in their 10w40 and 5w30).
All this information can be gleaned from Ed Hackett's oil FAQ (More Than You
Ever Wanted to Know About Motor Oil), which used to be posted to rec.autos.tech,
and is available at http://www.virtual-cafe.com/~john/AMSOIL/oilfaq2.html .
Ron
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