Let’s start by acknowledging “the moose on the table.” Every gearhead has an opinion about engine oils; each has a favorite oil that he or she knows with absolute certainty is “the best oil in the world.” Let’s also acknowledge that the confusion accelerated when oil manufacturers had to change oil formulations to deal with issues of 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 for synthetic oil in cars with the Flexible Service System (FSS).

The breadth and depth of available information about motor oil are 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-powered engines available 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 other 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. Should you feel you have been successful with your past oil change regime, continue as you have been doing.
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 key points on the subject.

Engines and engine oils

Oil requirements have been changing as engine and emissions technologies have changed. Engines built in the last 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). What this means for the oils you use is that they can be lower viscosity because there is less oil contamination from blowby through the gaps and lower wear because of the alloys used. From 1976, with the introduction of catalytic converters in gasoline engines, engineers were built to tighter tolerances than before, with some alloys, but there was still a concern about blowby and wear. These engines required oil with higher viscosity and higher levels of zinc and phosphate (referred to generally as ZDDP – zinc dialkyldithiophosphates) that helped reduce wear on critical bearings and camshafts/tappets.

Prior to 1976, engines were built to somewhat looser tolerances and wear was a significant concern. Though Mercedes engines were more reliable than most, engines routinely wore out at lower mileage levels. They required higher viscosity oils to provide protection at highway speeds, but usually got little protection upon startup prior to multi-viscosity oils being introduced. High levels of ZDDP mitigated wear, but because catalytic converters suffer early failure with traditional amounts of ZDDP in our engine oils (about 1,100 to 1,500 ppm), ZDDP use in the lightweight oils required by modern engines equipped with catalytic converters has been decreasing to now only approximately 600 ppm.

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 just on 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. SN is far better than oils of 50 years ago, but not as good as CI oils. The current diesel oil is classified CJ. The CJ rating doesn’t indicate an improvement in performance, but rather an EPA-mandated change in formulation.

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. Too much of the stuff in the oil increases deposits, ash, and sludge. Thus we do not recommend putting a ZDDP additive 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 good for use in either diesel or gasoline engines.

Do avoid using any oil label 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, use of other kinds of oils with high ZDDP levels in engines with catalytic converters will cause premature failure of the catalytic converter, a very expensive proposition.

For maximum protection, high quality synthetic oil is the best answer 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. This group of oils are usually formulated with higher-quality base-stock oils whose inherent shear strength is more important than a few parts per million of ZDDP. Descriptions such as “severely refined” or “hydroprocessed” 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 they are 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 spent by oil companies in getting it 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 created by those who own in-house oil chemists? Not!

Operational considerations

The correct viscosity or thickness of the oil is your primary consideration. 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 by the use of polymer additives, and the second their 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 up.

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 to 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 up contaminants in the oil and hasten engine wear.
Most of us do not have oil temperature gauges, hence we have no knowledge about 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 only within a very limited oil temperature range, usually between 160°F and 250 °F.

Points to keep in mind

It is a myth that you can’t mix oil brands or viscosity. It is 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 and will deep clean the engine slowly with repeated oil changes. Start with a couple of quarts of high quality synthetic oil and increase the amount of synthetic with each oil change.

The oil filter and air filter are just as important as the type and viscosity of the oil, since 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 away from the surfaces of the engine. 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 claiming to be “semi-synthetic.” There is no established standard for its 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 has been thoroughly warmed up, in order to have all of the contaminants 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 builder’s 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, use high quality mineral-based oil until oil burning stops, then switch to synthetic oil.

Any oil change interval is just 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 Performance Analysis Company ([email protected]) operated by MBCA member George Murphy.

In Conclusion

In conclusion, based upon input from numerous experts with their own specific passions and recommendations for “the perfect oil,” here are some guidelines you can use to get 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 15W-40, reasonable for our climates.

•    Use CI-4/SL oils in pre-1975 gasoline-engine classics.

•    Continue to use the manufacturer-recommended oil (usually CI-4/SL in diesel-engined classics.

•    Use high-quality synthetic oil in 1976-1997 gasoline engine classic with the CJ-4/SM classification to protect the catalytic converter.

•    Use quality oil and pay the necessary premium over bargain oil (oil is inexpensive compared to 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 with the higher viscosity oils that are driven more than occasionally, 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 edited by Richard Simonds, based on primary input by Albrecht Stachel, and contributions from Ken Adams, Pierre Hedary, and Jonathan Hodgman, as well as online references of the American Petroleum Institute, the Society of Automotive Engineers, and Richard Widman at www.widman.biz/English/.