Cars by name
Trucks and Jeeps

Engines / Trans
Repairs / Fixes
Tests and Reviews

Which oils are right for older engines? Zinc, ZDDP, and more

Ben Deutschman of the Slant Six Club of New York and New Jersey wrote that while the lack of ZDDP is probably not a critical issue for well worn engines, it is a serious problem for newly rebuilt ones. He suggested using additives from Studebaker Technical Products (STP) or others to compensate for the lack of ZDDP, especially if you have a newly rebuilt engine. He also pointed to the importance of re-lubing in a rebuild.

Under new governmental emission control regulations, the anti-wear additive ZDDP (zinc­dialkyl-dithiophosphate) and a related family of similar zinc-salt additives (ZnDTP or ZDP and others) are being severely reduced in the newest passenger car engine oil so that the zinc and phosphorus will not damage new catalytic converters. The first oils to be affected would be those engine oils with the SM designation; this designation was introduced in 2004.

SM oil was designed for the most modern engines, though designated as "For all automotive engines currently in use." The question remains, “will this newest oil be detrimental to our older engines?” The answer: we don't think so, but...

Unfortunately, we are the dinosaurs of the automotive industry, and we don't represent the real motivation for changes, improvements and modifications in chemicals for passenger cars. At this time there are no simple, definitive answers, but all is not lost. The questions involved in engine oil and our special-need engines took me to the industry experts: the American Petroleum Institute (API).

The American Petroleum Institute is composed of members of the oil industry, including the well-known oil companies like Shell, BP, and Exxon, as well as the lesser known but equally important companies producing OEM and aftermarket specialty-use products, additive and chemical manufacturers, automobile builders, oil industry equipment suppliers and many others directly and indirectly involved with oils. Together these many and diverse groups have agreed to set standards to which they hold themselves.

There are two primary types of lubrication found within an automotive engine. The first is "hydrodynamic lubrication". This is the 'wedge' lubrication formed by the movement of the parts (bearing and shaft, for example) which builds a layer of lubrication between the two parts and replenishes this layer as the parts continue to move. (See Skinned Knuckles, July 2006, pages 35-37 for an explanation of the 'oil wedge'.) The other type of lubrication known as "boundary lubrication" is the lubrication of two parts without the formation of a full film of lubricant. This type of lubrication often requires the addition of special anti-wear materials to reduce friction.

That is a very important point. Not all oil companies or product manufacturers must belong to API or even subscribe to their services. But those that do, and those that meet the standards - and they are many and complex - may display the API trademarks on their product. It is illegal for an oil company (or retailer) to use the trademarks if they are not licensed to do so by API. This gives the API the ability to confirm that the product does meet the standards set by the industry. So stringent are the standards, and so rigorous is the testing, that the API trademark replaced the need for military specifIcations for oil products. Military specs were discontinued for oil in the early 1970s.

In addition to just issuing the trademarks, the API also has watchdog and policing powers to control the use of the trademarks and take legal action against those who chose to use it illegally. The legal steps could go as far as a recall of an illegally or improperly labeled product.

"Donuts" or "Starbursts" are not just given out upon request. To qualify for an API trademark license, an applicant must subject their product to a series of extensive industry tests. These tests include bench testing (laboratory analysis) and engine testing (actually being run in engines in a test facility under strictly controlled parameters). These tests can be extremely expensive, beginning at about $350,000 each - assuming that the product passes on the first try!

API does not do the actual testing. There are a number of independent, certifIed test facilities, and many of the oil companies have their own certified testing facilities. API does not rely merely on the word of the testing facility, but requires that they test according to a set of rigidly controlled standards and guidelines and that they regularly test 'reference oils' to validate their results.

The industry agrees on a regular basis as to the manufacturers, types, sizes and designations of engines to be used in the tests so that the results are uniform. Engines used are from production runs and are not modifIed to enhance individual tests. Although an oil company using an independent testing facility might specify which facility, they cannot specify which engine, or banks of engines are to be used for their tests. Upon com pletion of the tests, the results are often sent to API for analysis, or API may request specifIc results. Often the testing labs will just report the results to API to qualify for (or be denied) a trademark.

Both the laboratory tests and the engine tests are spelled out in pages of requirements, but just to give you some idea of what they test for, we will list sever­al of the test categories.


1. viscosity tests - all that apply, typically SAE 5W30 and 10W30. Must meet standards set by pre-determined parameters.
2. foaming tests - four sequences to target standard parameters.

  1. phosphorus tests
  2. deposit tests
  3. homogeneity and miscibility testing
  4. volatility plus a host of others ....

ENGINE TESTS include :

  • rust testing
  • viscosity increases over 60 hours
  • piston skirt varnish
  • weighted piston deposits
  • cam plus lifter wear (note specific tests for cam and lifters)
  • cam wear
  • stuck rings (hot)
  • oil consumption
  • low-temperature viscosity performance
  • engine sludge
  • rocker arm cover sludge
  • engine varnish
  • oil screen clogging
  • stuck rings (cold)
  • oil screen debris
  • bearing weight loss
  • plus a long series of viscosity grade testing.

The point is, the industry has set pretty stringent standards for themselves; the standards do not favor one manufacturer over another or one type of oil over anotµer. Each category noted above might require a separate test, so it is not a quick process. The formulation of the oil, and again each oil company has its own 'secret' formulas and special additives, must meet minimum standards. An oil can exceed those standards, and often the advertising of a particular oil will feature the fact that one oil is superior to another in a particular additive. But bottom line is that all oils that meet API standards are good.

Have any of the oil companies or auto builders tested specifically for 40, 50, 60 year old engines? Of course not. But they have tested for, and are continuously testing, for cam and lifter wear. Is this to say that we will not suffer damage in those areas in the future? Again, of course not, but we have to go with the best information that we currently have. And frankly, SM oil, with reduced zinc and phosphorus has only been around for less than two years. No one has tested adequately, and under controlled conditions (to our knowledge), for excess wear with our older engines under SM oils. It would take a lot of driving for that much wear to occur in just a couple of years, and then there is no scientific evidence that the lack of zinc/phosphorus specifically was responsible for any damage that might have occured. Cams and lifters have been suffering in-engine damage for as long as they have been used in engines.

The grading of oil is uniform throughout the country. An oil graded, for example, 5W30 and sold in a traditionally very cold geographical area (Northern Wisconsin, for example) will be the same in performance as a 5W30 oil sold in Southern Florida. The additives might be slightly different, enhanced for the colder climes or the warmer ambient temperatures, but the oil will react similarly.

Do you want to know more about classic cars? Allpar recommends Skinned Knuckles magazine. No frills, no glossy color photos of muscle cars, just practical tips for restoring classic cars, repair guides, and other invaluable information for the classic owner.

Often (one survey indicated almost 20% nationwide) oil is sold by convenience stores or auto parts stores without the API trademark but with a statement in the text on the bottle or can that the oil is "API-SA". That is, it meets the standards that the API has set for SA oil. To meet the SA standard, a product needn't be much more than just base oil. It does not have to have any additives, it does not have to have passed any testing, it does not have to be clean-burning, or varnish resistant, or have corrosion inhibitors. I wouldn't recommend it, but SA is probably what was used in the 1920s/early 1930s. But SA used in a more modem engine could be disastrous. And oil problems might take thousands of miles to become evident. Once the damage is done within an engine, mechanical repairs are the only way of reversing them. Check that oil that you picked up along the road on your last trip, or that oil that you bought cheaply at a swap meet. Make sure that it carries the proper API grade designation for your car.

Still on the topic of oil problems is the question of mixing oils. Can you mix Shell 10W30 and Valvoline 10W30 in your engine? Probably yes, with no ill results. Modem oils are formulated to be compatible with other oils. Can you mix a 5W30 with a 20W50 oil? Theoretically, yes. But now the question is, what do you have in your engine? Does it meet the qualifications required for a 5W cold-temperature oil or for a 20W cold-temperature oil? Is the mix a 30 or 50 viscosity hot temperature oil? You cannot arbitrarily state that because· you used a 50%/50% mix the two oils that they will average a 12W/40 oiL It doesn't work that way. First of all, the base oils may not blend with each other and they might not provide the boundary lubrication that you need in the cams and lifters. The components that make an oil a multi-weight might not be compatible with each other, even though both grades of oil used were of the same brand. The chemicals could easily change their characteristics and not do the lubrication that one oil or the other would have done. In an emergency, sure, get oil into the engine, but don't run it for any length of time or under adverse conditions. Get it out of the engine as soon as the emergency is over, and put the proper grade of oil into the engine, up to the 'Full" mark on the dipstick.

We were supposed to be discussing ZDDP. Why did I go off on such a tangent? Well, it was intentional. You realize that oil is subjected to grueling and intensive testing. One of the purposes is to see that oil lubricates properly. Remember the explanation of SM oil? It reads, "For all automotive engines currently in use." It doesn't exclude engines manufactured before 1940 or any other year. It reads "For all engines .... " API was asked this very question and rather than paraphrase their response, I am going to print it as received.

I talked with the OEMs in Detroit. The API SM IILSAC GF-4 engine oils should be suitable for older engines with flat fappets. The specification testing for oils with this performance category includes two engine wear tests (Seq. IIIG and Seq. IV) which use engines with flat tappets. Both the Seq. IIiG (GM engine) and Seq. IV (Nissan engine) are flat tappet engine tests measuring wear. For an oil to meet the specification these wear and scuffing tests must be passed. You will remember that API SM/GF-4 engine oils have a phosphorus limit of 0.06% min. and 0.08% max. It is also important to use the recommended viscosity grade (e.g. SAE 10W-30, SAE 10W-40).

An alternate suggestion is for those who are not convinced they may wish to use oils meeting the new API CJ-4. Although designed for diesel engines it has lower ash levels. Phosphorus levels are as much as 0.12%. API CJ-4 engine oils should be showing up in stores shortly. It may be possible to find these oils in SAE 10W-30 as well as the [other grades].

While it isn’t as big a problem as some say, it is still an issue, especially for those with newly rebuilt engines.

Is the lack of zinc and phosphorus as big a problem as it appears to be? No! Emphatically, no! So many of the on-line chat rooms are talking about the new oils being harmful or even dangerous to our older engines. Look at the source. Everyone "knows someone, who knows someone” whose engine was supposedly destroyed by modem oils. I question that. If all of these new urban legends were true, there would, be a mountain of destroyed engines to rival Mount Everest. It only takes one person to yell “fire!” with conviction and the entire theater empties out. Well, someone yelled "fire" about engine oils and started a stampede for the exit.

The new oils have been tested, and passed more stringent testing than our engines are going to receive. Granted, after the fact, ZDDP, zinc and phosphorus have been touted as the only things between our engines and total disaster. Zinc and phosphorus were excellent anti­wear additives, and they are still available in diesel oils. But they have been replaced by other additives in SM oils. They have been tested in independent laboratories and the results analyzed. The new additives provide adequate anti-wear protection.

Then there is the question of mixing high zinc/phosphorus content oils with SM to give "necessary" protection. There is no guarantee that a quart or two of 'older' oils, SJ or SL or even a specially formulated high ZDDP-content oil, will homogenize with SM oils to produce the correct or desired level of zinc/phosphorus for older engines.

Finally, there is the question of additives. Reading the labels on oil additives in a local auto parts store yielded only one which specifically mentioned zinc. We cannot guarantee that it will properly blend with your SM motor oil, nor can we guarantee that it won't settle out of solution and just lie at the bottom of your oil pan. This would be even worse than knowing you are not properly protected; it would give you a false sense of security in thinking that you've covered all bases. It might not be so. The testing required for engine oil does not include testing with aftermarket additives or miracle-fixes.

We queried a group of research scientists who have studied ZDDP and published work on the subject. Although they can identify the strengths and weaknesses of ZDDP and other various anti-wear additives, they, too, are reluctant to define any particular after-market additive as a replacement for zinc/phosphorus. And though they can identify a possible alternate to ZDDP, they feel that without proper and adequate testing, it's anyone's guess as to whether it will work:

"We are scientists who look at these things at a molecular level with the help of computer simulations. Once we understand how things work at a molecular level, you'd still need real-life testing to know which product works and which one doesn't. We can only give guidelines as to how to come up with new products. However, I would expect that some of the molybdenum-containing additives (in particular, MoS2-containing additives) may play the role that the ZDDP decomposition products play. They are sometimes used in tuning to reduce the friction in the engines, but it should also form anti-wear pads. Again, this would require testing."

It appears, then, that although there may be some potential for additives to supplement the anti-wear properties of SM oil, until they are thoroughly tested and approved we can only speculate as to whether they are doing the job in helping to decrease wear within the engine.

diesel crossover oils for zincIf you still have reservations about using SM oils in older engines, there is a pretty safe alternative for you: diesel oil. At the present time, diesel oils do not have the same restrictions on zinc or phosphorus as gasoline engine oils. Take a careful look at diesel oils at your local auto parts or chain store. The API donut will specify the type of service that the diesel oil is designed to handle. Many diesel oils are "cross-over" oils - they carry a dual designation on the donut (for example, "API Service CI-4/SL). The "C 1-4" is the diesel rating, but the "SL" is the gasoline-engine rating. They have incorporated all of the additives to make their oils compliant with gasoline engines without sacrificing proper lubrication in a diesel engine.

If there is one drawback in using a cross-over diesel oil in a gasoline engine, it is that the ash content will possibly be higher than a gasoline-engine only oil. In a well-maintained, tight gasoline engine it shouldn't present too much of a problem.

Straight diesel oil (not a cross-over formula) could be used in a pinch, but crossover oil is so easy to find that there shouldn’t be a problem on that account. And although for gasoline engines, only oil grades SJ, SL and SM are current, for diesel cross-over oils, SH grade oil is still permitted.

To summarize, there are methods of solving the boundary-lubrication problems in our older cars. But as you see, a bottle of modern engine oil is a complicated blend consisting of nature's own oil, miracle chemicals, high-technology, serious laboratory and engine testing and, I suspect, just a pinch of a witch's brew thrown in for good measure.

From Skinned Knuckles, October, 2006. Thank you, Neil Maken, for making this available!

Newest Ram Built to Serve models honor the U.S. Air Force

Former Ram chief engineer Michael J. Cairns
2021 Ram 1500 Rebel navigation screen
What’s new for ’21? The big list of changes

More Mopar Car
and Truck News