Use the type of motor oil specified in your customer's owners manual.
Most manuals say it's okay to use a variety of viscosity grades
depending on temperature conditions. Generally speaking, the
following holds true:
- 10W-30 is best for all engines for year-round
driving. 10W-40 is more popular in the aftermarket, but 10W-30 is
actually a better oil because the additive package in it holds up
better over the long haul. This is why General Motors does not
recommend 10W-40 motor oils for any of its cars.
- 5W-30 is approved for most late-model
four-cylinder, V-6 and V-8 engines on a year-round basis. It is not
approved for many turbocharged or diesel applications, some high
output V-8s, or applications involving driving at sustained highway
speeds or towing in hot weather. It may not be the best choice for
older, high mileage engines. 5W-30 is used as the factory fill oil on most
new cars because it pumps through the engine more quickly after
start-up (important for keeping overhead cams properly lubed). It
also makes cold weather starting easier and reduces fuel
consumption.
- Straight viscosity oils have limited
temperature ranges and lack the versatility of multi-viscosity oils.
They can be safely used as long as their temperature limits are
observed.
- Straight 10W is okay for cold weather starting
and driving, but too thin for warm weather driving.
- Straight 20W
is okay for all around driving,
but doesn't provide the temperature protection of straight 30W
(which gets too thick at low temperatures for easy cold starting).
- Straight 40W and
50W oils are primarily for
heavy-duty applications.
- Special multi-viscosity oils such as
2OW-50 are
typically formulated for racing or severe duty applications such as
towing.
- Synthetics are a good alternative for any of
the above because most provide extended temperature protection and
service life.
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