Should we be using lighter weight oil?

George Harris harchris at
Sun Dec 29 14:42:44 EST 2002


The multigrade oils have 2 ratings; The first number is the viscosity of
the oil at normal temperatures (maybe room temp?) and is a good
indication of the rate of flow when the engine is cold. The second
number is the viscosity of the oil at working temperature, so a number
of 50 means that it has a viscosity of a 50 weight oil when at working

This question was asked recently so there was a fairly technical
discussion which is probably captured in the archives, but I read
nothing in that discussion that changed my thinking on the topic.

The most wear on our engines occurs when we first start them up because
the oil has drained down and the typical operating cushions of oil
haven't had the time to build up to operating pressures. When the oil
pressure has built up there is a cushion of oil between all the main
bearings,  connecting rod big end bearings, and camshaft bearings.
Beyond that I think most of the engine is lubricated by spray and drip.
So the most important thing for the longevity of our engines is to get
oil where it is needed as quickly as possible. This dictates a low first
number since it is harder to pump the heavier weight oils when cold.

After the engine is warmed up we want an oil that has enough body to
maintain that cushion and that dictates a high second number.

In the good old days, before synthetics, a 10W30 was a good compromise
because the standard oil couldn't have a much larger range. But today
with synthetics, and with a 15W50 it sounds like you are using a
synthetic, it is possible to have a much wider range than before. My
suggestion would be to use a lower first number in the winter, perhaps a
0, and a slightly higher first number in the summer. Do they recommend
10 for the summer oil?

But if you are using a synthetic why not use 0W40 (or 0W50 if available)
year round? If 10W30 is recommended then you've got it covered :-)


> I have a '91 20V Coupe in the northeast with 20 degree winters and 90 degree
> summers. I noticed Audi and just about every manufacturer is moving towards
> 0W30, 5W30 type lighter weight oils. Should we be doing the same?
> I currently use Mobil 15W50 year-long, changed every May and November.
> What's your opinion on moving to lighter oils?
> Should I change oil to something lighter in the winters or all year long, such
> as 10W30 or 5W30?
> Thanks,
> Chris

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