"K. Smith" wrote in message
...



The multigrade oils only thicken up when they get really hot (various
additives, plastics etc expand & change as the temp rises, but the base
oil is the lower claim, so a 20-40 is actually 20 grade oil with additives
to help when it gets hot), as in a properly thermostatted car engine.


K: Either you have just enough information to be dangerous or you badly
mis-stated what you were trying to say.

All oil, single or multigerade thin as they heat up. Multigrade oils just
don't thin out as fast.

If by "plastics" you meant "polymers" then part of what you said is correct.
A multigrade starts with a base that is comparable to the lower weight
number and adds polymers that react with heat. This reaction slows down the
thinning action, it does NOT reverse it.

What most people do not realize is that the two numbers are given from
entirely different tables. There is a range of viscosities that is covered
under the "W" range and another range of viscosity for non-W. An oil rated
at 75W has about the same viscosity as an oil rated at 10.

Another important aspect of the rating is the fact that SAE motor oil is
rated at 100 degrees C. When you need the viscosity this is the
temperature that you oil is likely to be at.

A multigrade oil allows you to have a higher viscosity oil at 100 degrees C
that you can still manage to pump out of the oil pan when the temperature
drops well below freezing. It comes at a price, however. The polymers tend
to be fragile, and can be damaged by excessive heat or contamination. A
20W-40 is a much more robust oil than a 5W-40 because it started with a
heavier oil and doesn't need as much treatment to stretch it out.

Putting additives in a multigrade oil is also taking a risk for the simple
matter that it may be impossible to tell how the additive might react with
the various polymers in the oil. I doubt that the additve manufactures
fully test against every possible brand and weight of oil, and even if they
did I doubt even more that they would let you know if they discovered a few
brands that reacted badly with their additive.

If you live in Florida then you really don't need to worry about multigrade
oils. Run 40 weight and you are fine. If you live in Bismark North Dakota
in the winter time a multigrade oil is exactly what you need.

Rod McInnis