In several important ways, classic car engines are different than modern car engines. They are lower revving, they have larger metal-to-metal tolerances, and often have cork or rope seals. They also require more anti-wear additives to prevent premature wear. For these reasons, the best oil for classic cars requires a little more thought than just grabbing your favorite brand off the parts-store shelf there is engine oil choice.
The primary issue is that the phosphorus in ZDDP is hard on catalytic converters, and exhaust gas recirculation (EGR) systems perform better when there are more detergents and dispersants in the oil. So ZDDP has been reduced, while detergents and dispersants have been increased in modern oils.
For our classic cars, these changes can have a compounding effect. ZDDP is added to oil to coat and protect high-wear surfaces, like the solid-lifter valve trains featured in many classic cars. Increased detergent levels tend to wash away the little ZDDP that is still attempting to protect our valve trains. Finally, modern oils have more friction modifiers to increase fuel efficiency, which can lead to reduced oil film strength and damage parts.
When these oil changes first started occurring, the primary alternatives were diesel oils or racing oils, which contained higher levels of ZDDP than conventional oils. Each of these presented their own problems. Diesel oils have plenty of ZDDP, but unfortunately even higher levels of detergents than conventional oil. Race oils have a good balance of ZDDP and detergents but lack the anti-corrosion additives that are critical to keep internal corrosion low in little-used classics.
Today, several specialist oils companies have come out with oils formulated for classic cars. These oils balance the ZDDP and detergent levels adequately and include appropriate corrosion inhibitors to keep our engines from damage whether they are running or in storage.
What Is ZDDP?
Zinc Dialkyl Dithiophosphate, or ZDDP, is a zinc-based anti-wear additive that effectively helps lubricate engines. It is physical and functional properties make it ideal for use in oils and greases to help control oxidation and corrosion.
If you have spent any time reading or listening to stories about changes in oil over the past decade, you are probably confused, scared, or both. The issue is that changes in oil formulation have reduced the amount of a Zinc additive and often increased the amount of detergent in modern oils. For a while there were few oil products available for classic cars, but today, several companies have come out with products specifically designed for older cars.
The ZDDP content was lowered in conventional motor oil to prevent the possible fouling of the catalytic converters. Apparently, the zinc and phosphates contained in it coats the active substrate in converters, which may contaminate it and decrease its life expectancy.
All major oil manufacturers, to comply with the EPA, have followed the American Petroleum Institute’s (API) guidelines and lowered their zinc content.
Levels for ZDDP in oils used to be 1,200 to 1,400 parts-per-million (ppm). Since the early nineties, manufacturers have been reducing this number. The latest automotive oils, rated API-SN, are around 800-900 ppm.
Late-model engines with roller camshafts have considerably less camshaft lobe friction, so they do not require high levels of zinc.
Without the correct amount of ZDDP, older, push rod-style motors will wear prematurely. The damage may not happen overnight, but over months and years, the lack of protection will prematurely wear cams, lifters, and other internal parts.
Classic Car Oil
Most vintage and classic automotive engines have a push rod-operated valvetrain driven by a flat-tappet camshaft. To prevent premature cam and lifter wear, there must be an adequate amount of anti-wear additives. This was not a problem 30 or 40 years ago, but for environmental reasons, the quantity of an additive called ZDDP has been reduced significantly in modern oils.
Engine Break-In
During the break-in period of a freshly rebuilt engine, lack of proper lubrication can be fatal to internal parts, especially camshaft and lifters. After rebuilding an older engine, either add a ZDDP supplement or use a racing oil, which has a higher level of phosphorous in it. After the engine break-in period you can go away from the racing oil if you like.
Engine Oil Break-In Additive
Most engine and engine component manufacturers recommend a zinc and phosphorus content of more than 1,200 PPM for break-in, and most will void warranties if this minimum is not found in the oil sample you supply when returning broken parts for warranty.
High Zinc Oil for Classic Cars
Today, many of the well-known oil companies are offering oils formulated for vintage and classic cars. These oils balance ZDDP, detergent levels, and corrosion inhibitors.
Diesel Oils
Diesel oils have around 1,000 ppm in ZDDP content, but also have high levels of detergents. Too much detergent in a non-diesel motor could do more harm than good.
Race Oils
Race oils, with 1,200 ppm, offer a good balance of ZDDP and detergents, but lack anti-corrosion additives. These corrosion inhibitors protect engines from damage whether they are running or in storage. Since most classic cars do a lot of sitting and relatively little driving, keeping internal corrosion to a minimum is crucial to engine longevity.
What Weight Oil Is Best?
When choosing the best oil for classic cars, the question of viscosity (thickness) always comes up. Since engine start-up is where most wear occurs, the obvious answer is to use the lightest weight oil available. If you live in an extremely hot climate, use the second lightest weight oil.
Are Synthetic Oils Worth the Extra Money?
Some people sleep easier knowing they have bought the “best” and “most expensive”. Amsoil 10W-40 AMO gives all the benefits of synthetic oils and contains ZDDP anti-wear agents. However, it is significantly higher in price of the conventional motor oils. Whether it is worth it is up to you, but first consider the blow-by and loose tolerances of older engines. It is probably more cost-effective to use conventional oil and change it regularly.
What Is the Best Oil for Classic Cars?
The best choice for giving your engine the correct amount of phosphorous and zinc anti-wear additives it needs are motor oils with high ZDDP content already in them (between 1,200-1,400 ppm).
Finally
Use caution when adding ZDDP additives. Follow the manufacturer’s specifications and measure with care. Too much is just as bad as too little or none. Add a ZDDP additive to your regular engine oil when you change oil, many manufacturers offer motor oils especially made for classic cars. Motor oils specifically blended for vintage cars often will contain a ZDDP additive.
Some manufacturers now offer synthetic oil that includes high amounts of zinc as an additive. Generally, synthetic oils with high zinc packages exceed the standards needed for classic car and vintage cars, no matter how you use the car.
Classic car oils need changing every 3,000 miles or annually depending on your driving frequency. Choose an oil containing a ZDDP additive for your classic car. Normal oil, with additives already in it should work simply fine if they also contain ZDDP or use your regular oil and add a ZDDP supplement or another anti-wear protective supplement that does not contain chlorinate paraffins.
