10W-30 better than 5W-30?
Magic2626
Since 1992, the factory-fill oil in Corvette engines, except '92 LT5s, has been
a synthetic, 5W-30. Prior to that, as far back as the late-1970s, GM used a
petroleum-based 5W-30 in Corvettes, except '90-'91 ZR-1s.
The reason GM recommends 5W-30 is it's the politically correct choice when fuel
mileage is a prime consideration. The U.S. Environmental Protection Agency,
which oversees fuel economy standards for cars sold in this country and
believes 5W-30 oil is a significant energy conservation measure, pressures car
companies into 1) using 5W-30s in new vehicles and 2) marketing them to service
customers. In response to this pressure, many years ago a policy was made by GM
at the corporate level that, with few exceptions, the recommended oil for all
passenger vehicles be 5W-30.
When you read the "Maintenance and Lubrication" section of the 1997 Corvette
Service Manual carefully, you soon find there are alternatives. The book states
on page 0-47, "The recommended SAE engine oil viscosity is 5W-30, however, if
the temperature is above -18ĄC (OĄF) 10W-30 may be used." Not only is this true
for C5 but the same or similar language appears in owner's or service manuals
going back a decade or more.
While we dispose of used oil and filters responsibly, recycle our cans and
plastic and keep our room air conditioners set for 78Ą, the Technical Staff of
Vette Magazine cares little about the fraction of a percent better fuel mileage
we supposedly gain with 5W30 oil in our Corvettes. It is our opinion that
high-performance engines regularly operated in above 0ĄF outside temperature
and in their intended duty cycles will maintain performance longer if an oil of
different viscosity range is used.
Any 5W-30 engine oil, including the synthetic in C5s, uses chemical additives,
called "viscosity-index (VI) improvers", to widen its viscosity range enough to
make it a 5W-30. VI improvers contribute little to lubrication and can make a
"multi-vis" oil less effective at high temperatures than an oil without them.
This can become a problem when the oil is under high shear loads at
temperatures above 225ĄF. It becomes a potentially damaging problem when oil
temp. approaches 300ĄF. If your oil temperature in the pan is 250ĄF or above,
bet that oil temp in bearings is approaching 300Ą. Near the piston ring lands
and the upper sections of the cylinder walls, it is that or above.
Another characteristic of a 5W-30 that can be a problem in severe service is a
relatively high evaporation rate at high temperatures compared to lubricants of
more narrow viscosity range. A higher evaporation rate contributes to increased
oil consumption and substandard lubrication in areas of high temperature.
There are no VI improvers in quality, 10W-30, synthetic engine oils and their
high-temperature evaporation rate is less. Thus, 10W-30 synthetics are better
lubricants in severe duty applications. For that reason, we recommend that any
C5 owner, and, for that matter, the owner of any Corvette built since GM began
telling us to use 5W-30Ńput a synthetic, 10W-30 oil in their engine.
A good choice is Mobil 1, 10W-30 synthetic. An even better choice is Red Line
Oil's 10W-30. While it is a good product, to keep its cost reasonable, Mobil 1
is made with a synthesized-hydrocarbon, base-stock. Red Line uses a more
costly, but higher-performing polyol-ester base stock as used in jet engine
lubricants. Because of this, Red Line 10W-30 due has slightly better
anti-friction characteristics and is capable of lower oil temperatures at a
given ambient temperature and engine load. Additionally, at oil high
temperatures during severe duty operation, Red Line offers more scuff
protection when lubricating main, rod and cam bearings.
The second oil selection issue to address is the Service Manual statement that
LS1s operated in North America use only an oil meeting GM Standard 4718M. The
Manual also states that oils meeting 4718M might be hard to find outside North
America and that LS1s operated in such places can use any 5W-30, 10W-30 or
10W-40 petroleum or synthetic oil meeting APIĘSJ and Association des
Constructuers Europeens de l'Automobile (AECA) Standard 3-96.
In a survey of some mass-marketers selling major brand synthetic oils to the
DIY, we found synthetics may or may not be labeled as meeting GM 4718M but all
meet the American Petroleum Institute's (API) Service SJ and International
Lubricant Standardization and Approval Committee (ILSAC) GF-2.
Using information provided by the the GM Powertrain Division and the Lubrizol
Corporation, a chemical manufacturer that sells raw materials to the world-wide
oil refining industry, we researched these different oil specs.. GMPT supplied
the particulars of 4718M and Lubrizol's web site (www.lubrizol.com) had the
test sequence requirements for API SJ, ILSAC GF-2 and AECA 3-96.
SJ and GF-2 have only minor differences, none of which affect the ability of
oils meeting those specifications to lubricate. 4718M and 3-96 are similar,
however, both differ from API SJ and ILSAC GF-2 in two critical measures: 1)
the GM and the AECA standards require better performance in high
temperature/high shear environments, two conditions present in any engine
generating very high oil temperatures and 2) both 4718M and 3-96 require a
lower oxidation rate, also a necessity in an engine generating high oil
temperatures. Those differences mandate synthetic oil because petroleum-based
oils cannot meet the higher requirements. Better high temperature/high shear
numbers and lower oxidation rates are characteristic of all quality synthetic
oils.
The Vette Technical Staff suggests C5ers looking to buy oil for their LS1s not
worry so much about 4718M and look for a major brand of 10W-30 synthetic oil
meeting API SJ or ILSAC GF-2. Examples are Red Line, Mobil 1, Castrol Syntec
and Havoline Formula3 Synthetic. Those operating their Corvettes outside North
America should use an oil that meets AECA 3-96.