There is no viscosity cure all. Viscosity choice is heavily dependent on the owners ability to conduct research for the intended application while resisting the temptation to follow the most popular herd...
Some choose to limit their research by just asking Billy, Joe, Bob or Bubba, and then accept the answer as the unquestionable gospel truth.
Others may come across BITOG only to find the same thick vs. thin questions and opinions. Many then become members and decide to join (or graze with both thick and thin herds) here at BITOG!
IMPROVED HIGH RPM PROTECTION
The viscosity seen in a bearing or cam may be
completely different than the labeled viscosity.
Petroleum oils lose considerable viscosity at high
RPMs and high temperatures. In contrast, Red
Line synthetics are much more resistant to viscosity
loss than even the best petroleums. A petroleum
20W50 begins as a 20W oil and the oil is
thickened with a polymeric plastic substance which
will thicken the oil at higher temperatures.
Unfortunately, when the oil enters a high shear
stress area such as a bearing, these large polymer
molecules align themselves in order to create the
path of least resistance. As shown in Figure 1, the
apparent viscosity can be much less than the
viscosity listed on the container - typical 20W50s
will actually be similar to an SAE 30 or 40 in the
bearing. The same behavior occurs with all multigrades,
with a petroleum 5W30 shearing down to a
10W or 20W and a synthetic 5W50 shearing
similarly to a petroleum 20W50. No wonder 5W30s
are not recommended for sustained high-speed
driving.
These polymeric thickeners are very large
molecules which will suffer from thermal cracking
when exposed to high temperatures. When these
molecules crack, they reform to create varnish
deposits which can stick rings and plug turbo
passages. For this reason many turbocharged
cars recommend against using wide-range multigrades.
Since Red Line Synthetic Oils satisfy the
high-temperature and low-temperature flow
requirements of a multigrade using only a fraction
of these thickeners, all viscosity grades are
suitable for high temperature and turbo use.
VISCOSITY GRADE SELECTION
The proper viscosity grade to use depends on
many factors such as ambient temperature, miles
on the engine, bearing and piston clearances, and
type of service. A general rule of lubrication is to
use the lowest viscosity possible which will provide
the proper separation of metal. Anything more than
this minimum will increase power loss due to
friction and will reduce the pumpability of the oil at
all temperatures. However, one must consider
synthetics differently when determining viscosity
requirements. At high speeds, a petroleum 5W30
can appear to be a 15W in the bearings due to the
shear stress, yet Red Line 5W30 will actually be an
SAE 30 in the bearings. There is a significant
difference in lubricating an engine with a 30
compared to a 15W. Red Line Synthetic oil will
provide better viscosity protection than the next
higher petroleum viscosity grade. If clearances are
tight and very little lugging occurs, then the 5W30
or 10W30 should be adequate. Less turbo lag will
be noticed with the 5W30. If ambient temperatures
will regularly climb above 100°F, then the 10W40
would provide an additional safety margin. If the
engine is air-cooled, or if the engine is older and
has greater bearing and ring clearances, the
15W50 and 20W50 will develop a thicker oil film. If
very low temperatures occur, the lower viscosity
grades, (5W30, 10W30, or 10W40) will flow better
and lubricate the engine more quickly after start-up.
http://www.redlineoil.com/whitePaper/motoroils.pdf
