At 10:43 PM 6/29/2000 -0300, Eugene Balinski wrote:
>Could someone please remind me about motor oil ratings ? In particular,
what is the "W" for in 20W50 ? As I remember, it means that the oil is 20
weight in the Winter (or cold wx), and 50 weight when warm, gradually
changing from one to the other. In this manner, it flows more easily when
it is cold. Is this correct ?
The "W" does stand for Winter, but the oil does not get thinner as it gets
colder. All motor oil (at least those selling for under $10/qt) will get
thinner as it gets hotter. It's just that the multiweight oils exhibit a
lesser change of viscosity with temperature.
Straight weight oil does thin out a lot when it gets hot, so you may need a
minimum of 30 weight to protect your engine in hot weather. However, this
same oil gets so thick (viscous) when cold that you may not be able to
start your engine when the temperature is below 0dF in the winter. In past
times the solution was to change to 20 weight, or even 10 weight oil in
very cold weather, and then switch back to 30 weight again when the weather
warmed up.
Now consider 10W30 oil. This stuff has all the lubricating cabability of
30 weight when it's hot, but it doesn't get so thick when it's cold. As a
result you can use it in place of the straight 10 weight oil you may
otherwise have used in cold weather. Thus the rating 10W30, meaning 10
weight for winter and 30 weight otherwise. It simply doesn't change
viscosity as much with temperature, so you can use it all year round.
Now suppose you have an older engine and want a heavier oil, say 40 or 50
weight oil for hot weather use. These straight weight oils can get very
thick in just moderatly cold temperatures, like around the freezing point
of water. Under those conditions it may not adequately lubricate your
engine for a time just after cold start (assuming that you can start it at
all) for lack of circulation. For this we have 10W40 and 20W50 multiweight
oils, so we can have our heavier weight oil for summer protection and still
be able to start and run the car safely at lower temperatures.
And the term "multi-weight oil" is commonly misconstrued. The
"multi-weight" or "multi-viscosity" oils in fact have the least change in
viscosity, while the straight weight oils have the greatest change in
viscosity with temperature changes. The term "multi-weight" is intended to
apply to the multiple applications of the oil for use in different
temperatures, not to the actual viscosity of the oil.
One last note. There are some modern synthetic oils that are rated at
10W50, 5W50, and even 0W50, which are very nice to use in extremely cold
climates.
Barney Gaylord
1958 MGA with an attitude (and 10W40 oil all year in northern Illinois)
http://www.ntsource.com/~barneymg
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