Classic Car Oil - Do Old Engines Need Zinc Additive

Do Older Cars Need Special Oil?

Engine oil has changed quite a bit in the last 20 years. It has changed so much that simply using the wrong oil for old cars can seriously damage your engine, especially if it is an older engine with a flat tappet camshaft. If you have a newer car, then there isn’t much issue selecting an oil, as most of the oil on the shelves meets the current API service ratings, but this isn’t the case for older engines or classic cars with original engines. There are specific issues at play that you need to be concerned with when choosing the best oil for older cars.

Pretty much all modern OHV (Overhead Valve) gasoline-powered engines use roller cams, where the lifter that rides on the cam lobes has a rolling follower that protects both the cam and lifter from significant wear. This does not fully eliminate the need for Zinc and Phosphorous in the engine oil, but it does allow the oil to carry significantly less in the formulation. The current 2020 API oil standard is SP, which is limited to 800ppm of ZDDP, which goes back to the 2010 SN rating. This is enough to protect a modern engine with roller cam followers and modern metallurgy and tolerances, but not for use as vintage muscle car oil. Put that same oil in a rebuild or new crate engine with a flat tappet cam, and you will have a worn cam inside 30 minutes of break-in.

One side note to this conversation is OHC or “Over Head Cam” engines. More modern engines are an OHC, either SOHC (Single Over Head Cam) or DOHC (Dual Over Head Cam) design. What is different from a pushrod style engine, where a single camshaft is located in the center of the engine block, is that the OHC valves are operated directly off the camshaft. In the case of SOHC engines, there are rocker arms that follow the cam lobes, whereas most DOHC engines use a flat tappet cap on top of the valve itself. Unlike pushrod engines, there is less pressure in most stock applications to necessitate higher ZDDP levels. If you have one of these engines and use heavier valve springs, you may need a higher level of ZDDP. Once broken in, these engines do not need 1200 ppm of ZDDP, 800 is enough. All engines regardless of type, need higher levels of ZDDP for break-in.

For classic cars built before the late 1980s (1987 was the first year for GM to use a roller cam, 1985 for Ford in Mustangs), however unless you have installed a later-model engine or done a roller-cam swap, you most likely have flat tappet camshaft and a ZDDP level of 1200 is the minimum level for adequate protection.

What Is A Flat Tappet Camshaft?

Simply put, a flat tappet camshaft means that the lifters have a flat solid surface on which they ride along the surface of the cam lobe. While we say flat, it is actually slightly convex (domed). This convex shape facilitates rotation of the lifter, which provides a smooth uniform surface for the life of the lifter and cam. This means that the two metal surfaces slide along each other, which generates a lot of heat and wear over time.

Do I Need ZDDP For My Classic Car Oil?

Oil provides protection for the two surfaces, acting as a barrier so the two metals don’t actually grind on each other. Once the surfaces reach 300 degrees F, however, the lubricity of the oil breaks down and is no longer protecting the parts. You might say “well, my temp gauge reads 210 when I am driving, so I’m good,” but this is definitely not the case. The temp gauge refers to the engine coolant, not the temperature of the internal parts, and absolutely no localized temperatures where friction occurs. Within minutes of start-up, the cam and lifters can reach this critical temperature.1. list text here

ZDDP, which is essentially a complex series of zinc and phosphorous molecules, provides protection that oil doesn’t in two ways. Cold-start protection is enhanced through the residual layer of ZDDP that is left on the metal as the oil cools after shutdown, so you have a thin layer of protection for that initial startup where there is no oil pressure. The primary ZDDP function, however, occurs when the oil reaches 300 degrees. Around 250 degrees, the ZDDP begins to activate and provides the lubricity needed when the oil has broken down from heat.

With older engines built with lesser quality metallurgy and flat tappet cams, the recognized minimum level of ZDDP is 1200 ppm, but as mentioned before, API-rate SN and up oils have a maximum of 800 ppm, which is not enough. The fast answer here is yes, your classic car engine does need a high-ZDDP oil or SN rated oil with a ZDDP additive. The best oil for old engines will have a minimum of 1200 ppm of ZDDP.

Synthetic Or Conventional: Best Oil for Classic Cars

While this issue isn’t directly tied to ZDDP levels, it is a valid question that is often asked. The truth is that when considered viscosity to viscosity, there is no difference in the initial lubricity of the two oil types. Note that word “initial”, as this is the key to differences between synthetic and conventional oils. The main difference is that synthetic oil does not break down over time unlike conventional oil. Technically if kept clean, synthetic oil can be run forever, but in an engine that is impossible. This is why newer vehicles have 5, 10, and even 20k mile oil change intervals, because synthetic oil lasts longer.

One reason why some automotive enthusiasts do not like synthetic oil in classic cars is because it is thinner. Compared weight to weight, synthetic oil is thinner than conventional with the same viscosity rating. This can lead to issues such as oil leaks. Because synthetic oil is thinner on a molecular level, it can seep through spaces where conventional oil cannot. Additionally, the detergents tend to clean away more of the old built-up gunk inside the engine, exposing more potential leak points than if the engine contained conventional oil. For rebuilt low-mileage engines, synthetic is the best motor oil for older cars, but if your engine has a lot of miles on it, a conventional high zinc oil for classic cars might be the better option because synthetic might be too thin.

Synthetic oil will also lower your oil pressure, because it is thinner. The standard oil pressure minimum is 10 psi of pressure for every 1000 rpm, but with synthetic oil the standard minimum is 5 psi. So switching to synthetic may give you a scare, but as long as you have 5 psi per 1000 rpm, it is just you classic car motor oil, not your engine.

Whether or not to run synthetic or conventional oil is completely up to you, synthetic over time offers more protection of the internal components due to its resistance to break down and its ability to stay clean longer. In the end, synthetic oil is better for most engines at the correct viscosity, but if you have an old engine that might have a leak or 5, switching will most likely expose more leaks over time.

How Do I Add ZDDP To My Classic Car Engine?

You have a couple of choices for maintaining the correct levels of ZDDP in your engine’s oil. You can buy an oil specified for older vehicles or use an SN rated oil and add more ZDDP to it. The generally accepted target for ZDDP content in classic car engine oil is 1200 ppm, even though most pre-bottled Hot Rod motor oils have up to 2100 ppm of ZDDP. Some experts claim that at some point above 1500 ppm of ZDDP, your engine will experience the opposite of protection, as higher levels of ZDDP in your engine will cause pitting. It is a fine balance between protection and damage. We asked Chris Barker, senior manager of the tech services department at Royal Purple to help relieve some confusion of varying levels of ZDDP. “ZDDP is a category of additive not a specific additive, just like ‘shoe’ is a category of thing, rather than a specific thing; different shoes accel at different uses and there are varying levels of quality across all shoes; lubricant additive chemistry is the same,” Barker told us.” The chemistry restrictions (reduced anti-wear additive) defined by oil specification (API, ILSAC, GM dexos, etc.), primarily concern phosphorus which is typically only found in the ZDDP additive.” This means that an oil advertising 2100 ppm of ZDDP most likely does not contain enough to cause metal pitting, because each oil brand’s formulation of ZDDP is different. This is why the common statement about too much ZDDP is “at some point above 1500 ppm” because there are just too many factors to be considered for a blanket statement of “this is too much ZDDP.”

Pre-bottled engine break-in oils tend to have much higher levels of ZDDP, as much as 4000 ppm, which could cause serious damage to your engine if run long term. Most break-in oils are designed for a maximum of 400 miles of use. An engine oil such as Lucas Hot Rod & Classic High Zinc oil is a perfect choice for your classic, as it contains 2100 ppm of ZDDP. This oil also contains a blend of additives that provide long-term protection for extended periods of non-use, as most classic cars don’t get daily driven. Lucas recommends changing the oil every 2000-3000 miles. Higher ZDDP levels do tend to bring down the change interval according to our research.

The other option is to add a ZDDP additive, such as Redline’s Engine Oil Break-in Additive. Designed for use in new engine break-in, it can be used with each oil change as well at the proper mix ratio. A 16oz bottle of Redline Engine Oil Break-in Additive contains 2200 ppm of ZDDP, which treats 12 quarts of oil. For oil changes, Redline recommends 8 oz per oil change, giving you a boost of about 220 ppm for a 5-quart oil capacity, meaning an SN-rated oil will go from 800 ppm to 1020ish ppm, which is enough to adequately protect you classic car engine for years to come.