Does Synthetic Oil Cause Leaks?

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David Hilgendorf
by David Hilgendorf
August 29, 2022

What are seals?

Seals often take the form of rings or gaskets, which join two surfaces together and must create an effective fluid barrier over long periods of time under extreme changes in pressure and temperature.

Most motorists don’t think about seals until they notice a fluid leak. Common seal locations include valve covers, crankshaft, transmission output shaft and axles. Vehicles contain many different seals, which are responsible for:

  • Retaining fluids within the system
  • Keeping contaminants out
  • Keeping different fluids separated
  • Confining pressure

When synthetic lubricants were first introduced, many people erroneously believed the oils were too slippery and leaked past seals. However, field studies and real-world use by countless motorists has proven otherwise.

A more common scenario is that switching to synthetic lubricants will clean internal components, washing away existing sludge and other contaminants, which could potentially be plugging leaky seals. Using synthetic lubricants from the start keeps internals cleaner and helps seals last longer.

Age, temperature, mileage and even storage can lead to seals shrinking, cracking or tearing.

What are seals made of?

In the 1930s, engineers pioneered the manufacturing of synthetic rubber, also known as elastomers. Made primarily of elastomers, modern seal materials offer enhanced performance in harsh environments.

Common automotive seal materials include nitrile, neoprene, silicone, ethylene acrylic, polyacrylate and fluoroelastomer. Each material offers different strengths and weaknesses throughout different temperature ranges (see list below).

Age, temperature, high mileage and storage can lead to drying and hardening of elastomers, causing shrinking, cracking or tearing. In addition to fluid leaks, damaged seals can allow dirt and other contaminants to enter the system, negatively affecting performance and increasing wear.

Nitrile seals, better known as buna-N (NBR), are common in automotive applications due to their low cost and good resistance to oil, water, grease and other substances. However, NBR offers poor resistance to ozone and weather aging. Ethylene acrylic and polyacrylate are often used in transmission and power-steering units.

Proper seal lubrication keeps fluids in and contaminants out of your vehicle.

Do seals require lubrication?

Wherever seals are installed and whatever they are made of, proper lubrication is required to maximize life and performance. Seal and lubrication compatibility is achieved by selecting the proper base oils and additives that cause seals to swell at a slow, controlled rate over their usable lives. This swelling allows the seal material to fill any gaps caused by worn material and prevent premature leakage.

Seal conditioners are added to lubricants to help keep seal materials supple and prevent them from becoming brittle and causing leaks. For example, valve seals prevent oil from entering the cylinder during the intake stroke. A dry, brittle seal may allow oil to leak past the seal and burn during combustion, causing the engine to consume more oil.

What Does Oil Viscosity Mean

Oil viscosity is the measure of its resistance to flow. How quickly or slowly motor oil flows affects how well it protects your engine.

Learn More

Put to the test in a Las Vegas taxi cab driven over 100,000 miles in severe service, AMSOIL Synthetic Motor Oil and Automatic Transmission Fluid (ATF) delivered excellent protection for seals and offered reliable, zero-leak protection.

AMSOIL synthetic lubricants are formulated and tested to be fully compatible with seal materials in all of your vehicles and equipment. AMSOIL delivers excellent seal conditioning and protection, helping the seals function effectively throughout their service lives.

Common Automotive Seal Material Properties

Nitrile, Buna-N (NBR)

-50ºF ~ 250ºF
• Low cost
• Good resistance to petroleum oils, water, silicone oils, greases & glycol base fluids
• Good abrasion resistance, cold flow & tear resistance
• Poor resistance to ozone & weather aging


40ºF ~ 225ºF
• Resistant to both petroleum lubricants & oxygen
• Provides good resilience & flex resistance


80ºF ~ 400ºF
• Broad temperature range
• Good ozone resistance
• Resistant to compression set
• Low resistance to hydrocarbon fluids like gasoline or paraffin fluids

Ethylene Acrylic

40ºF ~ 300ºF
• Good resistance to lubricating oils, greases, transmission fluids, power steering fluids & diesel fuel
• Higher temperature limit than NBR • High/consistent vibration dampening capability
• Fair cold-temperature limit
• Costs more than NBR


-20ºF ~ 300ºF
• Good resistance to mineral oils, hypoid gear oils, EP additives, greases, aging & flex cracking
• Higher temperature limit than NBR
• Fair cold-temperature limit
• Lower mechanical strength
• Costs slightly higher than NBR
• Poor dry-running ability, water resistance


-30ºF ~ 400ºF
• Good high-temperature resistance
• Compatible with wide range of fluids
• Fair resistance to water & dry running
• Fair low-temperature resistance
• High cost


by David Hilgendorf

AMSOIL Technical Writer and 20-year veteran of the motorcycle industry. Enjoys tearing things apart to figure out how they work. If it can’t be repaired, it’s not worth owning.

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