The Weighty Subject of Oil for Old Cars
Classic Tech by Richard Simonds
Let’s start by acknowledging “the moose on the table.” Every gearhead has an opinion about engine oils; each has a favorite brand and type of oil and knows with absolute certainty it is the best oil in the world.
Let’s also acknowledge that confusion accelerated when oil manufacturers had to change formulations to deal with catalytic converters and changing engine technologies. Let’s understand that most of this article will focus on oils for vehicle engines prior to 1998. After 1998, Mercedes-Benz stipulated that low-viscosity synthetic oils should be used in all their vehicles in the United States. Let’s accept that oil-change intervals also have passionate adherents; recommendations vary from every 3,000 miles for non-synthetic oil in classic cars to as much as 15,000 miles in many new cars.
The breadth and depth of available information about motor oil is bewildering. There are currently roughly 480 companies certified by the American Petroleum Institute (API) to produce motor oil, with most companies offering 75-150 different formulations. To compound the confusion, manufacturers change oil formulations frequently. How is one to choose?
For those of us with Mercedes-Benz engines manufactured from 1998 on, there is little reason for confusion at this time. The company installs Mobil 1 05W-40 synthetic oil in all new cars, uses the brand at all dealerships, and recommends use of that or equivalent high-quality synthetic oils for non-dealer oil changes. Oil-change regimens must be respected. Even that detail is now monitored and prompted by your modern car. Clearly, more frequent oil changes improve the odds for an engine’s survival. If you’ve been successful with past oil change regimens, continue your routine.
The challenge with oil is mainly for those of us who operate Mercedes-Benz engines, both gasoline and diesel, older than 1998. The design and execution of these engines requires well-vetted oils for their long-term survival. Following are some key points on the subject from Star technical contributors.
Engines and engine oils
Oil requirements change as engine and emissions technologies change. Engines built in the past 14 years are built to very tight tolerances, using custom-engineered alloys in the blocks, cylinder heads, pistons, rings, and camshafts, and with lighter masses (e.g., pistons are about half the size they were in the past). In addition, rubbing parts have been replaced with rolling components in the critical valve train. What this means for the oils used in today’s engines is that they can be lower viscosity because there is less oil contamination from blow-by through the gaps and lower wear because of the alloys used.
Prior to 1976, engines were built to somewhat looser tolerances and wear between metal parts such as tappets and camshafts was a significant concern. They required higher viscosity oils to provide protection at highway speeds, but usually got little protection upon start-up prior to the introduction of multi-viscosity oils, so careful warm-up was required.
In addition, high levels of zinc and phosphorous additives (lumped under the common term ZDDP, short for zinc dialkyldithiophosphates) mitigated wear on metal parts rubbing together at high pressures, but oil still needed to be changed at intervals of less than 3,000 miles to remove the contaminants and metal particles that accumulated in the engines. Though Mercedes-Benz engines were more reliable than most, engines routinely wore out at lower mileage levels than today’s engines.
The changes began in 1975 with the introduction of government-mandated emissions reductions. In particular, catalytic converters were added to the exhaust systems on gasoline engines to mitigate carbon monoxide, unburned hydrocarbons and, later, NOx (nitrogen oxide) emissions. Additionally, engines were built to tighter tolerances than before, using increasing amounts of high-tech alloys to reduce blow-by and wear, but progress was gradual. Improved engine management systems also contributed to better performance, less wear on metal parts, and reduced oil breakdown. With the improved engineering, the oil companies developed synthetic oils that could maintain lower viscosities and last longer without breaking down. By 1998, the combination of improved engine design and improved synthetic oils prompted Mercedes-Benz and other manufacturers to recommend use of low-viscosity synthetic oil and longer periods between changes.
In addition, the catalytic converters contributed their own problems. Lead antiknock additives in gasoline were clogging the core substrate, so leaded gasoline was the first casualty in the battle against emissions. With more experience, engineers also learned that a catalytic converter’s lifetime was reduced by long-term build-up due to reactions between the phosphorous and the metals in the catalytic substrate.
Accordingly, oil companies began to phase out ZDDP additives in oil as rapidly as possible after 2006. Now it is common to find the low-viscosity oils designed for modern engines to have as little as 600 ppm of ZDDP, whereas oil used previously generally contained at least twice that amount. With little warning from the oil industry, and no effort by the industry to study what damage this change might have on older engines, engine builders and mechanics soon discovered that the tappets and cams on newly rebuilt engines failed almost immediately. Moreover, engines with many miles were showing accelerated wear on rubbing parts. However, once the source of the problem was identified, oil companies began to modify their products, increasing the ZDDP in high-viscosity oils used in pre-catalytic converter engines. Though there is considerable confusion, careful use of the information provided on the oil containers can help owners avoid problems.
Reading the Labels
The API performance classifications we see on oil container labels were first used in 1960. The classifications are based on the quality of the base oil and the additives, not merely the additives. Oil for gasoline engines is designated as “S” for spark and “C” for compression in diesel engines. As additive packages for oils improved, another letter was added to modify the symbol. The current gasoline engine oil is identified as SN and the current oil for diesel engines is identified as CJ. The CJ rating doesn’t indicate an improvement in performance over CI, but rather an EPA-mandated change in formulation. SN is far better than oils of 50 years ago, and generally now as good as similar-weight oil produced prior to 2006, but CI/CJ oils are at least as good and, in the view of some specialists, better for older engines.
The API states: “Each new API category has placed successively lower phosphorus and zinc limits, and thus has created a controversial issue of obsolescent oils needed for older engines, especially engines with sliding (flat/cleave) tappets…[and]…engines built before 1985 [that] have the flat/cleave bearing style system of construction. This reduction in anti-wear chemicals in oil has caused premature failures of camshafts and other high pressure bearings in many older automobiles and has been blamed for premature failure of the oil pump drive/cam position sensor gear that is meshed with camshaft gear in some modern engines.”
Fortunately, most oils in the viscosity range required for non-catalytic converter engines do have a reasonable amount of ZDDP. However, it should be noted that ZDDP does not activate until full engine operating temperature is reached. ZDDP is not the be-all and end-all of oil additives. Also, too much of the stuff in the oil increases deposits, ash, and sludge. Thus, we do not recommend adding ZDDP into the crankcase. The correct amount needs to be in the formula of the oil you purchase, not something you add to oil that was not formulated with it.
Diesel engines built before the current generation of clean diesels did not have catalytic converters, thus are permitted to have generous amounts of anti-wear additives such as ZDDP in the formulations. Oils appropriate for later clean-diesel engines often have several different rating classifications on their labels, indicating that the oil is appropriate for use in either diesel or gasoline engines. Avoid using any oil labeled with a starburst on it in an older engine. The starburst symbol signifies that the oil meets the reduced additive guidelines for catalytic converters. However, using other kinds of oils with high ZDDP levels in engines with catalytic converters will cause premature failure of the catalytic converter, a very expensive consequence.
For maximum protection, high-quality synthetic oil is recommended for 1976 through 1997 engines. Pre-1976 vehicles should continue using high-quality mineral-based oils with a CI-4/SL or CH-4/SL classification.
Synthetic oils are, simply put, better as a rule. These oils are usually formulated with higher-quality base-stock oils whose inherent sheer strength is more important than a few parts per million of ZDDP. Descriptions such as “severely refined” or “hydro-processed” are used to identify higher quality base oil stock. Look for these words, but expect higher prices for higher performance. Synthetic is more expensive, but the quality means oil changes can be cut in half – 6,000 miles between oil changes rather than 3,000 – so the price is pretty much a wash.
Aftermarket oil additives are to be avoided. Mercedes tells you not to do it, and the company is right. Any additional additive to the oil alters the chemical composition already provided by the manufacturer. There is no predicting what kind of alchemy is taking place in your engine with the introduction of additional chemicals, even if they are merely more of what is already in the oil. Think of the millions of dollars oil companies spend to get the formulation right, and you think that an impetuous moment in the automotive section of a store is going to provide you with a better oil than an oil created by those who employ in-house oil chemists? Not!
Operational considerations
The correct viscosity or thickness of the oil is your primary concern. The Society of Automotive Engineers has established viscosity ratings from 0 to 60, with lower numbers indicating thinner viscosities. Most oils sold today are rated with two numbers, the first indicating their winter or cold-start viscosity, enhanced with polymer additives, and the second represents viscosity at operating temperatures. For example, a 15W-40 oil flows at an equivalent rate of 15 at cold-weather start-up temperature, but maintains a viscosity of 40 when the engine is fully warmed.
The graph below illustrates the relationship between ambient temperature, oil viscosity and the range within which oil provides desired operating protection. Get the viscosity too high, and more wear and heat will result. Use a viscosity too low, and crankshaft-bearing life will suffer. Note that 50-weight oil only reaches the protective zone at high temperatures, so it is only suitable for very hot climates or very worn engines.
As an engine ages, a slight increase in viscosity is justified. But it is essential to choose oil that flows properly at start-up temperatures. The thicker the oil, the slower it travels. Disproportionate damage occurs upon cold starting (after 8 to 10 hours of rest), compared with extended warm operation. Until the engine reaches full operating temperature, there is very poor lubrication. This fact is of critical importance.
Warming the engine to operating temperature carefully but quickly is an essential habit for an engine’s survival. The general guideline for older engines is to start the engine and allow it to run for at least 15 seconds for the oil to fully circulate and then drive gently until the water temperature gauge moves, indicating that the thermostat has opened. This temperature threshold is not the green light to drive the engine aggressively. Engine oil does not really warm up enough until the car has been driven for half an hour; idling doesn’t count. If a car is started, it should be driven. Frequent starts and short trips build contaminants in the oil and hasten engine wear.
Most of us do not have oil-temperature gauges, hence we don’t know when oil temperature has reached 150 F. However, a good rule of thumb is to drive the car for half an hour. At that point, the oil should be up to operating temperature and all that can be expected in engine performance is available to us. It is also important to know that all oils provide optimum protection within a very limited oil-temperature range, usually between 160 F and 250 F.
Points to keep in mind
It’s a myth that you can’t mix oil brands or viscosity. It’s a myth that your engine will start to leak due to seal shrinkage if you change to synthetic oil. Virtually all oils have formulation ingredients that make them compatible with seals, and may actually reduce leakage with use. Avoid engine-oil rinses, or anything similar. High-quality synthetic oil has inherent detergency and dispersants that deep clean the engine slowly with repeated oil changes. Start with a couple quarts of high-quality synthetic oil and increase that amount with each oil change.
The oil and air filters are as important as the oil’s type and viscosity because the air filter is designed to capture airborne contaminants before they enter the engine and the oil filter is designed to remove engine residue and contaminants carried from the engine surface. No oil can compensate for contaminate ingestion due to a dirty or highly oiled air filter, or a dirty or poor-quality oil filter.
Be wary of oils that are labeled semi-synthetic. There is no established standard for their use. These oils could be of the lowest base-stock quality with a small percentage of synthetic oil added. Avoid oils claiming to meet the specifications for older engines. It means that only minimum requirements are met, usually with oil stock quality used in that period of oil specification. There are reasons why the next generations of oils were developed. The earlier specifications did not get the job done. Always change oil that is thoroughly warmed to ensure all of the contaminants are in suspension.
Of particular importance: Do not attempt to break in a rebuilt engine, especially one with a new camshaft, with synthetic oil. All arguments to the contrary are totally false. The ZDDP additive, working with ingredients in the engine assembly grease, is essential to build up and smooth down the metal surface on the camshaft lobes. Without those ingredients, the camshaft can be destroyed. For break-in purposes on older engines, companies such as Metric Motors use high-quality, mineral-based break-in oil for the first start-up, and recommend that the engine be retorqued and valves adjusted again after 500 miles, including an oil and filter change, and to use moderate throttle and varying engine speeds during the initial 1,000 miles to break in the engine properly.
Once an older engine is broken in, different mechanics have their own favorites, but the consensus is that a good-quality non-synthetic oil – such as Castrol GTX, Castrol GTX High Mileage, or Pennzoil Long Life – is excellent in older engines if changed at one-year or 3,000-mile intervals, whichever comes first. Other mechanics prefer high-viscosity synthetic oils used in older engines – oils that contain higher amounts of ZDDP. Mobil 1, Valvoline VR1, Brad Penn, and Red Line are brands most often mentioned in our research. Synthetic oils are more expensive, but can run for a year or 6,000 miles before changing, so are preferred for the person who drives an older car as a daily driver.
Any oil-change interval is a rule of thumb; the only way to conclusively determine whether oil needs to be changed is to analyze the oil, a method routinely used by operators of commercial-vehicle fleets. You can get a similar evaluation of your oil from a company like Performance Analysis Company ([email protected]) operated by MBCA member George Murphy. Based on input from numerous experts with their own specific passions and recommendations for “the perfect oil,” here are some guidelines in choosing an oil that will minimize wear in your pre-1998 Mercedes-Benz engine:
- Use a viscosity rating that will meet the ambient temperatures where your classic will be driven (refer to your owners’ manual for recommended viscosity ranges). In older engines, our technical contributors generally use 10W-40, reasonable for our climates, recognizing that the oil still won’t be at efficient operating temperature until the car is driven for about 30 minutes.
- Use CI-4/SL oils in pre-1975 gasoline-engine classics.
- Continue using oil recommended by the manufacturer (usually CI-4/SL in diesel-engine classics.
- Use high-quality synthetic oil with the CJ-4/SM classification to protect the catalytic converter in 1976-1997 gasoline-engine classics.
- Use quality oil and pay the necessary premium over bargain oil (oil is inexpensive compared with an engine rebuild or replacement).
- Use your favorite, “absolutely best oil in the world” (IF it meets the specifications in the bullet points, above)
- Change the oil at least once a year. In older cars that are driven more than occasionally and use higher-viscosity oils, change non-synthetic oil every 3,000 miles and synthetic oil every 6,000 miles. Change the filter at the same time.
This article was compiled by Richard Simonds, with primary input from Albrecht Stachel, and contributions from Ken Adams, Pierre Hedary, Jonathan Hodgman and Mike Elias, as well as online references from the American Petroleum Institute (www.api.org), Society of Automotive Engineers (www.sae.org), and Richard Widman at www.widman.biz/English/.