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Diesel oil vs Gasoline oil: what oil to run in old cars?

June 7, 2018
By

As technology has advanced, you would assume that oils and their ability to lubricate have improved; and they have, but that development is a double edge sword.

The agenda behind oil development is as much about lubrication as it is a battle against environmental impact, catalytic converter health, and fuel efficiency.

If you look at most gasoline branded oils these days you will notice the words “Energy conserving” or “Resource conserving” on the button side of the API symbol.

Phosphorus ZDDP and sulphur limits in API engine oil specifications have been decreasing over the years:

Even though ZDDP is considered one of the best anti-wear additives, there has been a push since the 1990’s to reduce its levels because of anticipated negative effects against catalytic converters.

Gasoline and Diesel oils are regulated separately, and therefore are permitted to have different levels of additives.

Gasoline API categories:

Category

Status

Service

SN Current Introduced in October 2010, designed to provide improved high temperature deposit protection for pistons, more stringent sludge control, and seal compatibility. API SN with Resource Conserving matches ILSAC GF-5 by combining API SN performance with improved fuel economy, turbocharger protection, emission control system compatibility, and protection of engines operating on ethanol-containing fuels up to E85.
SM Current For 2010 and older automotive engines.
SL Current For 2004 and older automotive engines.
SJ Current For 2001 and older automotive engines.
SH Obsolete CAUTION: Not suitable for use in most gasoline-powered automotive engines built after 1996. May not provide adequate protection against build-up of engine sludge, oxidation, or wear.
SG Obsolete CAUTION: Not suitable for use in most gasoline-powered automotive engines built after 1993. May not provide adequate protection against build-up of engine sludge, oxidation, or wear.
SF Obsolete CAUTION: Not suitable for use in most gasoline-powered automotive engines built after 1988. May not provide adequate protection against build-up of engine sludge.
SE Obsolete CAUTION: Not suitable for use in most gasoline-powered automotive engines built after 1979.
SD Obsolete CAUTION: Not suitable for use in most gasoline-powered automotive engines built after 1971. Use in more modern engines may cause unsatisfactory performance or equipment harm.
SC Obsolete CAUTION: Not suitable for use in most gasoline-powered automotive engines built after 1967. Use in more modern engines may cause unsatisfactory performance or equipment harm.
SB Obsolete CAUTION: Not suitable for use in most gasoline-powered automotive engines built after 1951. Use in more modern engines may cause unsatisfactory performance or equipment harm.
SA Obsolete CAUTION: Contains no additives. Not suitable for use in most gasoline-powered automotive engines built after 1930. Use in modern engines may cause unsatisfactory performance or equipment harm.

 

Diesel API Oil Categories:

Category

Status

Service

CK-4 Current API Service Category CK-4 describes oils for use in high-speed four-stroke cycle diesel engines designed to meet 2017 model year on-highway and Tier 4 non-road exhaust emission standards as well as for previous model year diesel engines. These oils are formulated for use in all applications with diesel fuels ranging in sulfur content up to 500 ppm (0.05% by weight). However, the use of these oils with greater than 15 ppm (0.0015% by weight) sulfur fuel may impact exhaust aftertreatment system durability and/or oil drain interval. These oils are especially effective at sustaining emission control system durability where particulate filters and other advanced aftertreatment systems are used. API CK-4 oils are designed to provide enhanced protection against oil oxidation, viscosity loss due to shear, and oil aeration as well as protection against catalyst poisoning, particulate filter blocking, engine wear, piston deposits, degradation of low- and high-temperature properties, and soot-related viscosity increase. API CK-4 oils exceed the performance criteria of API CJ-4, CI-4 with CI-4 PLUS, CI-4, and CH-4 and can effectively lubricate engines calling for those API Service Categories. When using CK-4 oil with higher than 15 ppm sulfur fuel, consult the engine manufacturer for service interval recommendations.
CJ-4 Current For high-speed four-stroke cycle diesel engines designed to meet 2010 model year on-highway and Tier 4 non-road exhaust emission standards as well as for previous model year diesel engines. These oils are formulated for use in all applications with diesel fuels ranging in sulfur content up to 500 ppm (0.05% by weight). However, the use of these oils with greater than 15 ppm (0.0015% by weight) sulfur fuel may impact exhaust aftertreatment system durability and/or drain interval. API CJ-4 oils exceed the performance criteria of API CI-4 with CI-4 PLUS, CI-4, CH-4, CG-4 and CF-4 and can effectively lubricate engines calling for those API Service Categories. When using CJ-4 oil with higher than 15 ppm sulfur fuel, consult the engine manufacturer for service interval.
CI-4 Current Introduced in 2002. For high-speed, four-stroke engines designed to meet 2004 exhaust emission standards implemented in 2002. CI-4 oils are formulated to sustain engine durability where exhaust gas recirculation (EGR) is used and are intended for use with diesel fuels ranging in sulfur content up to 0.5% weight. Can be used in place of CD, CE, CF-4, CG-4, and CH-4 oils. Some CI-4 oils may also qualify for the CI-4 PLUS designation.
CH-4 Current Introduced in 1998. For high-speed, four-stroke engines designed to meet 1998 exhaust emission standards. CH-4 oils are specifically compounded for use with diesel fuels ranging in sulfur content up to 0.5% weight. Can be used in place of CD, CE, CF-4, and CG-4 oils.
CG-4 Obsolete CAUTION: Not suitable for use in most diesel-powered automotive engines built after 2009.
CF-4 Obsolete CAUTION: Not suitable for use in most diesel-powered automotive engines built after 2009.
CF-2 Obsolete CAUTION: Not suitable for use in most diesel-powered automotive engines built after 2009. Two-stroke cycle engines may have different lubrication requirements than four-stroke engines, so the manufacturer should be contacted for current lubrication recommendations.
CF Obsolete OBSOLETE: Introduced in 1994. For off-road, indirect-injected and other diesel engines including those using fuel with over 0.5% weight sulfur. Can be used in place of CD oils.
CE Obsolete CAUTION: Not suitable for use in most diesel-powered automotive engines built after 1994.
CD-II Obsolete CAUTION: Not suitable for use in most diesel-powered automotive engines built after 1994.
CD Obsolete CAUTION: Not suitable for use in most diesel-powered automotive engines built after 1994.
CC Obsolete CAUTION: Not suitable for use in most diesel-powered engines built after 1990.
CB Obsolete CAUTION: Not suitable for use in most diesel-powered engines built after 1961.
CA Obsolete  CAUTION: Not suitable for use in most diesel-powered engines built after 1959.

http://www.api.org/products-and-services/engine-oil/eolcs-categories-and-classifications/oil-categories

 

Much of the process is described here:

Download (PDF, 1.73MB)

 

ZDDP is particularly important in old cars that have flat tappet engines; engines where the contact between the cam and the rocker arm is non-rolling or non-hydraulic. A reduction in ZDDP will increase the wear, and eventually lead to failure.

Unfortunately, new API categories for gasoline oils do not permit the needed levels that flat tappet engines need. There are options for “racing oils” and ZDDP products you can add to your oil, but often at a much higher price.

 

Diesel oils, are currently regulated to 1200 PPM ZDDP while the gasoline oils are regulated to 800 PPM; a difference of 50%.

This difference is allowed in diesel engines because diesel fuel has a much higher energy density than gasoline fuel:

This is also why diesel engines make more torque and have better efficiency. Diesel is 15% more dense in power per the same fuel quantity.

This higher density creates more demand on the engine (read: bigger explosion), and therefore heavier wear. As such, more ZDDP is permitted.

It is important to note that there are two different kinds of ZDDP, primary (C-8) and secondary (C-4). Secondary ZDDP activates, or begins to work, at a lower temperature, while primary ZDDP is more thermally stable at higher temperatures. Diesel oils favor more primary ZDDP because their expectation is that diesel engines will spend more time at their normal operating temperature.  While old gasoline oils favored secondary ZDDP as insurance against people who drove short distances without reaching normal operating temperatures.

 

Comprehensive history of ZDDP:

 

Download (PDF, 711KB)

 

Conclusion: Take advantage of the difference in regulations and run diesel oil in your old cars.Thankfully because diesel oil is normally purchased in higher quantities, it is also often less expensive.

 


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