Motor Oils

Motor Oils for Older Cars

The Motor Vehicle Air Pollution Control Act of 1965 set the first federal vehicle emissions standards, beginning with the 1968 models which required that automobiles produce less air pollutants.  In 1975 auto manufacturers started adding catalytic converters to comply.  As the engine design and materials kept changing, the lubricating oil went thru a long history of reformulation with the addition of different additives to comply with the newer requirements.  One of these was the additive “ZDDP”.

“ZDDP” short for Zinc DialkylDithiophosphates is an anti-wear and extreme pressure additive, its primary role is to prevent metal-to-metal contact between engine parts by forming a protective film with phosphorus performing the anti-wear function.  The zinc/phosphorus molecules react in the oil and release phosphorus-containing compounds that make a wear-resisting phosphate glass film on the critical surfaces of contacting engine parts. With most brands, this process also results in phosphorus being released with engine emissions into the catalytic converter, where it can poison the emissions system over time.

In 1970 the API (American Petroleum Institute) changed the oil classification system, "S" for gasoline & "C" for diesel use.  The API service designation is on the “Donut” on the container with the SAE viscosity grade.

The EPA banned leaded gas in 1996 for ruining the catalytic converters.

The stricter emission requirements on new cars required oil producers to gradually reduce ZDDP from motor oils, which was one of the causes of catalytic poisoning.  

 If you have an older car that was specified to use API SF (1988) or earlier motor oil, its design requires ZDDP in the oil.  That means that you should not use the current API SM, SN, or SL oil in your old car engine.

I did not include a discussion of the SAE viscosity weight as must older car engines use a SAE 30.  For your newer family car you need to refer to the owner’s manual to see what it recommends, IE  5W-30 SM.

Time Line for Reformulated Motor Oils:

SA (1930) motor oil has no additives, not for engines after 1930. 

SB (1931) not suitable for engines built after 1951.

SC (1952) not suitable for engines built after 1967.

SD (1968) not suitable for engines built after 1971.

SE (1971) ZDDP was level standardized - not for engines after 1979.

SF (1979) ZDDP level reduced.

SG (1988) ZDDP level further reduced.

SH (1993) ZDDP level further reduced.

SJ (1996) ZDDP level reduced to 0.1%. Do not use in 2001 or newer cars

SL (2001) motor oil should not be used in 2005& later vehicles

SM (2004 & Current) phosphorous limited to 0.08% & ZDDP to 0.06%.

SN (2010 & Current) backward compatible to 1988.

The newer oils SN, SM, SL, etc. are better than the previous listings; however, because of the reduced ZDDP additive in the newer oils they are not satisfactory for use in engines with “flat tappet camshafts” as they will cause excessive wear. 

Lubrication Definitions:

A.   Fluid or hydrodynamic

Where the moving surfaces are completely separated by a continuous film of oil and no wear of metal surfaces.  The lower the viscosity number results in less resistance to motion.

Ex: Crankshaft and camshaft bearings.

B.   Boundary

Where sliding metal surfaces are in contact with a small amount of oil between the surfaces.  Extreme pressure situation where there is a wiping action of the surfaces.  Ex: Camshaft lobes and cam followers.  Rocker arms & tappets.  Piston rings & cylinders.

Oil Additives:  These chemicals are added to improve the oil properties.

1.     Anti-foaming agents to keep the oil from turning into foam.

2.     Heat stabilizers to prevent viscosity breakdown & ashing.

3.     Detergents to keep contaminants in suspension & off metal parts.

4.     Extreme Pressure (EP) additives to increase wear protection.

5.     Dispersant agents to keep combustion particles from clumping.

6.     Additives to neutralize acids & by products of combustion.

7.     Additive to change the characteristics to a multi-viscosity rating.

8.     Wetting agents to keep the oil from running off hot surfaces.

9.     ZDDP which was required as a special EP additive for the sliding cam to cam follower interface in a non-roller type engine.  This additive was in the SE oils but was gradually reduced to 0.08% in the current SM & SN oils today.

These are some oils and additives that can be used in your older engines.

1.     ZDDP can be added to your regular oil, SL-SM bringing it to SF.

Not available in auto part stores, Amazon & other type stores.

2.     Amsoil Z-Rod API SL. Check with Jim at Flower’s Marina for info.

3.     Valvoline Racing Oil VR1. Available at O’Reilly Auto Parts

4.     Hemmings Motor News Synthetic SJ

The following motor oils claim they have ZDDP in them but do not specify the level of zinc or phosphorous.

5.     Quaker State Defy  API SL Available at Walmart, etc.

6.     Accel API SF  Available at Walmart.

7.     Mobil 1 API SL Available everywhere.

8.     Royal Purple HPS

9.     I saw other additives like STP & such on the part shelves that indicated ZINC on the container.  They do not list the percentage and on the rear of one container it stated that this additive contained no zinc – yet it had ZINC in big letters on the front label.

Note: There are some diesel engine rated oils that have some ZDDP; however, they have a higher amount of detergent additive and a higher viscosity which can cause oil starvation in the bearings at high rpms. The   higher detergent ratio reduces the friction reduction that the ZDDP affords in the oil.  Also the lower shear rating of the oil is not designed for the higher rpms and bearing size of a high performance gasoline engine.