In the early 1950s, the so-called MS sequence engine oil designations were developed, including API ML (motoring light), MM (motoring medium) or MS (motoring severe). These oils were finally defined in the late 50s by sequence tests that measured resistance to corrosion, wear and oxidation, as well as deposit control.

At the time, the original equipment manufacturers also had their house brands. There were concerns that these designations were too broad and made it difficult to tell whether or not an oil was really suitable for use in a particular engine.

Around 1970, the American Petroleum Institutes S categories came into force to make it easier to identify the performance level of the oil. API started with SD for then-current oils, moving through the alphabet to todays SN oils. It designated earlier oils (SA, SB and SC) as obsolete. However, API didnt include fuel economy measurements. The ILSAC GF series was introduced in 1992 and is the S series counterpart with fuel economy and restrictions on viscosity grades.

Okay, now that Ive laid the groundwork, here is the question: What would you recommend for that really awesome 57 Chevy that your regular customer loves so much? Well, Ive got the answer to that one, because I owned a 57 at one time and actually have the lube oil recommendations. Use SAE 10W-30 viscosity at all temperatures or the following monogrades depending on temperature: SAE 20W or SAE 30 viscosity above 32 degrees Fahrenheit, SAE 10W at 0 degrees or SAE 5W below 0 degrees. The recommended oil quality was API MS (API SC), and the oil change interval was 2,000 miles in normal service or less in severe service.

The question of the proper engine oil is more complex than you might think. It depends on how old the car is and the kinds of engines that were available when it was made.

I was invited to the Arizona Concours DElegance by businessman Larry Read. For those of you who dont recognize the name, Concours is a show of antique and classic cars. These autos range from a 1905 Renault to a number of mid-1960s American and European beauties such as Corvettes, Mercedes and Jaguars. For the record, Larrys car is a 1934 Hudson Terraplane. I have to say that I dont remember ever seeing one. In fact, I dont remember seeing very many of these vehicles on the road!

Many owners of true antique automobiles wonder what oil they should be using. Such cars are from an era prior to the common use of engine oil additives, so its a tough call as to whether or not an additive-containing oil would be satisfactory. I think that, generally speaking, modern engine oils should be okay.

Back in my Pennzoil days, this question came up enough that we prepared a listing of older vehicles and our recommendations (primarily SF quality oils). Sad to say, I didnt keep a copy of the list, known as Todays Lubricants for Yesterdays Automobiles. I had one hope for backup: Mike Maddox. Mike was part of the Pennzoil Technical Service team, and a great resource for me. I took a chance and sent him a message. To my great joy, Mike had an electronic copy, which he sent to me.

Pennzoils list is similar to ones you may have seen from Gousha Chek-Chart (now part of Motor). It includes not only engine oils but also all the other major lubricant requirements for most American-made autos up through 1935. The engine oil recommendations are all single grades and vary by the season. For Larry and his Terraplane, the recommendation is SAE 30 in the summer and SAE 10W or SAE 20W-20 in the winter.

Thinking about lubricants for these beauties, you have to remember that additives didnt come into use until 1930. From 1930 to the early 1940s, very few additives were used, and they were mostly pour point depressants. During World War II, there were some pretty strong restrictions on personal car use, and the lubricants being developed were for the war effort.

In the mid-1940s, things began to change. First, oils that were developed for military use began to be offered in the marketplace. In 1947, the auto industry introduced the MS Sequence tests that form the basis of many of our current engine tests. API then introduced the first oil category system, API 1509. Additive chemistry began in earnest at this point.

Things moved along without too much difficulty until the advent of lower zinc dialkyldithiophosphate products. This occurred around 2000 with the introduction of API category SJ, which limited the amount of phosphorus in the oil to a maximum of 0.10 percent by weight. The driver behind this was protection of catalytic converters. Phosphorus tended to poison the catalyst and make them non-functional. Prior to API SJ, phosphorus levels in engine oils were about 0.12 percent, and in some cases even higher.

After these oils hit the market, classic cars began to have wear problems. The specific problem that Im sure many of you have heard about is that vehicles with a flat tappet design needed a higher phosphorus level than the available SJ oils provided, in spite of the API guidance that each category of engine oil would be backwards compatible with earlier categories.

A flat tappet in the hy­draulic valve lifter assembly makes contact with the cam lobe and can result in some pretty significant wear. Enough phosphorus will minimize this problem, and the necessary level seems to be around 0.12 percent. API SJ oils were low in phosphorus, which most mechanics refer to as zinc, since that is the source of phosphorus in engine oils. Some rumblings among the classic car groups were heard.

API SL and ILSAC GF-3 retained the lower level of phosphorus, but API SM/ILSAC GF-4 upped the ante by lowering the phosphorus limit to 0.08 percent by weight. This is the point at which the classic car folks became really upset. They wanted an oil that would protect their prized cars, but that, for the most part, wasnt available.

Meanwhile, some of the oil companies were hearing about this and recommended heavy duty engine oils, which were all around 0.12 percent phosphorus. That seemed to solve the problem, especially at break-in, and is why the AERA (Engine Builders Association) recommends diesel engine oil.

Many OEM engine folks believe the evidence is overwhelming that there is not enough ZDDP in GF-4 and GF-5 oils for older engines with flat-tappet cams. Some classic car guys did not reach 200 miles on high-dollar rebuilds before rounding off a couple of cam lobes, and others suffered early-mileage cam failures.

So we know that lack of ZDDP leads to wear problems in certain classic car engines, especially those with flat tappets. Sounds like if a little is good, then more must be better, right? Not so fast. Bob Olree, formerly with GM, says that too much can be a serious problem, too.

Olree cautions that large overtreatments, like what classic car mechanics might get if they were to add ZDDP to their engine as an aftermarket product, could cause problems later. Although break-in scuffing is reduced by using more phosphorus, longer-term wear increases when phosphorus rises above 0.14 percent. And, at about 0.20 percent phosphorus, the ZDDP started attacking the grain boundaries in the iron, resulting in camshaft spalling.

So, the bottom line for classic car enthusiasts is to make sure you are using an oil with the proper level of ZDDP, and beware of over-treating to the point that corrosive wear failures can occur. The best recommendations seem to be heavy duty engine oil for your classic car, and SAE 15W-40 and straight grades for those antique cars where viscosity is the real name of the game.

Happy motoring. (I think someone has already used that tagline.)

Industry consultant Steve Swedberg has over 40 years experience in lubricants, most notably with Pennzoil and Chevron Oronite. He is a longtime member of the American Chemical Society and SAE International, where he was chairman of Technical Committee 1 on automotive engine oils. He can be reached at steveswedberg@cox.net.