Choosing Motor Oil For An Antique Car Engine

A reader asks: What factors should be considered when choosing a motor oil for an antique car such as the Ford Model A?

Our answer: When it comes to choosing motor oil for vintage cars like the Ford Model A (built between 1927 and 1931), there are a few factors to consider. Today’s latest motor oils are designed to be compatible with modern emission components. Some traditional motor oil ingredients have been reduced in order to maintain compatibility with emissions equipment. As vintage engines obviously do not have today’s emissions equipment, motor oils meeting the very latest industry specifications may not be the best choice.

First and foremost, these antique engines tend to have flat tappet camshafts. Flat tappet cams require a motor oil with high ZDDP (zinc dialkyldithiophosphates) content. We will also refer to this as simply “zinc” for short. Motor oil without adequate levels of ZDDP can promote accelerated flat tappet cam wear. Modern motor oils contain about half of the ZDDP levels of motor oils from 25 years ago.

The second factor to consider is that vintage engines may have looser tolerances. Therefore, choosing a motor oil with adequate viscosity is required.

Third, antique vehicles tend not to be daily drivers. As they may only run a few times per month, choosing a motor oil with a robust anti-rust and corrosion package is sound insurance.

What About Adding A Zinc Additive To Regular Motor Oil?

Simply adding a ZDDP additive to any motor oil off the shelf may seem like a practical option on paper. But there is more to the story.

Motor oil makers spend millions of dollars formulating a motor oil product. Finding the correct balance of base oils and additive ingredients requires thousands of hours of lab and field tests.

This balance can be disrupted by adding an aftermarket oil additive. It may actually detract from the motor oil’s overall abilities. Zinc is simply one part of the equation. Adding an ZDDP additive to attain 2500 ppm of zinc may actually not be as effective as a properly formulated motor oil that has 1400 ppm.

Using an aftermarket additive is pouring an unknown factor into a carefully formulated and balanced formula.

Is Non-Detergent Oil Necessary For Vintage Engines?

There are two distinct lines of reasoning regarding the use of non-detergent motor oils in antique engines.

First, as only non-detergent engine oils were available when these cars were produced, the belief is that it is prudent to carry on with non-detergent oils. We disagree with this theory. Capable motor oil detergents and dispersants can keep engines internals clean and hold contaminants in suspension. Preventing the formation of varnish and sludge is a plus in any mechanically sound engine.

The second motivation for choosing a non-detergent oil is the concern that motor oil detergents could dislodge contaminants in engines with severe sludge. This may be a valid concern in engines that are in poor repair or that have been neglected. However, for most any mechanically sound engine, motor oil containing detergents is nothing but an asset.

Is Straight-Grade Oil A Must For Older Engines?

Should multi-grade motor oil be avoided in antique engines? Several decades ago, there may have been a valid reason for sticking with straight-grade oils. That is definitely no longer the case today.

The theoretical knock on multi-grade oils is that they may contain an ingredient called “viscosity index improvers” (VII). These VII additives are polymers that expand as the oil temperature rises. VII ingredients keep the oil from thinning out excessively at hot operating temperatures. VII additives perform only this task and add nothing else in terms of lubrication or protection. More VII additives may be used in multi-grade oils that have a long stretch from the winter weight such as a 0W-30 or 5W-40.

In the early days of multi-grade oils, the issue with viscosity index improvers is that they eventually break down. This would potentially lower the viscosity of the oil over time and possibly promote the formation of engine deposits. The chemistry has improved dramatically the decades, so that this a non-issue now.

Less VI improvers are used in synthetic multi-grade motor oils. Synthetics inherently have a linear viscosity across the temperature spectrum. There are also other factors to consider such as the quality of the VII additives and the overall competence of the oil formulation itself.

The good news is that high-quality multi-grade motor oil technology now offers the very best of both worlds. They provide both exceptional cold-start oil flow and steadfast shear stability at high operating temperatures over long intervals.

Are Any Of the “Newer” Formula Motor Oils Suitable For Antique Engines?

Using modern passenger car motor oils that meet the very latest industry specifications will not be a disaster in the short-term. But do keep in mind that motor oils meeting the latest industry specifications will have roughly half the zinc content of motor oils from yesteryear.

That does not mean that these oils are completely neutered of anti-wear additives. They employ modern base oils and ashless additive technology which performs exceptionally well in today’s cars and trucks.

In our opinion, using a motor oil containing at least 1000 ppm of ZDDP will provide a reliable and effective anti-wear barrier in classic engine valve-trains. There are motor oils on the market that are designed for older vehicles and are not constrained by the latest industry specs.

It should also be noted that the latest diesel engine oils (meeting the  API CJ-4 or CK-4 spec) also have lower zinc levels. Diesel engine oils holding the API CI-4+ specification (for pre-2007 models) may contain adequate zinc levels.

Are Synthetic Oils Safe In Antique Engines?

Synthetic motor oils can safely be used in any mechanically-sound vintage engine. By mechanically-sound, we mean no significant leaks or massive internal sludge issues. Synthetic oils are compatible with all seal and gasket materials. In fact, synthetic oil can preserve seals and keep them supple. Synthetic oil will not cause or exacerbate leaks in older engines. Today’s synthetic oils are also compatible with the remnants of any conventional oil that may be left in the crankcase when switching to synthetic from mineral oil.

Our Best Motor Oil Solution For Vintage Engines

We are highly partial toward a motor oil series in our product line. It ticks all the boxes for the protection and operation of vintage engines. This product is AMSOIL Z-ROD Series. It is available in 10W-30, 10W-40 and 20W-50. Here are the features and benefits.

• Contains 1440 ppm of zinc and 1320 ppm of phosphorus for steadfast flat tappet cam wear protection.
• Has powerful anti-rust and corrosion inhibitors for extended storage protection.
• Withstands extreme heat. Provides exemptional viscosity stability.
• Provides instant cold-start oil flow.
• Protects against the formation of varnish and sludge.
• Maximizes performance by fighting friction.
• Compatible with all gasket and seal materials.

Questions? Feel free to contact us using the form on this page or call us at 1-800-748-5781.