Oil Changes

Scott Harrison

"Growing old is mandatory, growing up is optional"
Staff member
Contributing Member
I get asked this question a lot. So I asked Jerry Carlson, our fuel/lubricant specialist from Ethyl Corp the following question:

For those people who only drive their Cobras 3000-5000 miles year, what interval do they need to change their oil? Does it differ if they use regular or synthetic oil? Assume a normal drive is 10-100 miles each time they take the car out. Car stored in garage.

This is the response I received. Some of this is enlightening:

The limited amount of driving suggests an annual oil change interval would be sufficient, but knowing how your friends are probably looking for an excuse to work on their cars I’ll dig a little deeper. Oil breakdown can be a pretty complicated mechanism. In some cases 10,000 miles barely stresses the oil while in others, 3,000 miles completely breaks it down potentially affecting engine life. The factors I would consider when making a recommendation include: average trip length, fuel metering system, ambient temperature, and driving style. To be safe I would recommend changing the oil every six months, in the Fall and in the Spring.

- Short trips are about the worst kind of service as you accumulate a lot of moisture in the fuel in the oil that never has a chance to boil off. Water and fuel combine to dilute the viscosity and promote oxidation of iron surfaces. Lower viscosity in key areas like bearings and camshafts lobes can promote premature wear. Contaminates in the oil will also lead to rapid depletion of key additives and cause the oil to break down. In extreme cases distinct layers of water and oil can form in the sump with water being on the bottom closest to the oil pump pick-up. This means that on initial start-up the first thing the bearings will see is a slug of water. In some recent cold start-up testing using fuel containing ethanol we accumulated over 1 quart of material in the sump in 48 hours!

- The type of fuel system is important because carburetors tend to over-fuel and have unequal fuel distribution throughout the cylinders, leading to fuel dilution of the engine oil. Because it’s a continuous process of fuel addition and removal there tends to be more fuel in the oil with a carburetor than with a closed-loop fuel injection system because carburetors are not as efficient with fuel metering. As mentioned earlier, fuel will readily mix with the oil which lowers the viscosity, and in the case of E10 causes water to accumulate in the oil. Ethanol loves water and will combine with the moisture that’s coming through with the blow by.

- Ambient temperature is a factor because it promotes condensation of moisture in the crankcase and extends the time necessary to warm-up the oil. In some cases it can take 30 minutes to achieve optimum oil temperature for removal of accumulated water and fuel. Very few drivers have trip lengths of over 30 minutes in normal service. The source of moisture comes from the combustion of fuel and resulting blowby. While piston rings do a great job of sealing the combustion chamber there’s always a small amount of gas that blows past the rings and ends up in the crankcase before being evacuated by the PCV system. Studies of this gas show it to have a composition that is between 50 and 80% water with a pH of 1 or 2. Anyone who has taken a high school chemistry class will recall that a pH of 7 is considered neutral with lower numbers being more acidic. Short trips and low ambient temperature are a one-two punch to any engine and will lead to an early demise.

- Driving style is important if the car is driven hard most of the time. This will cause shear-down of the viscosity modifier, permanently reducing viscosity. The viscosity modifier is what allows the oil to be both “10W” and “30” at the same time. It is comprised mostly of rubber that can be torn apart under extreme use. Hard driving also causes the fuel system to over-fuel, leading to accumulation of fuel in the oil and further decreasing viscosity. Even sophisticated close-loop fuel injection systems that employ O2 sensors in the exhaust will over-fuel when the going gets tough. When designing a fuel system you always err on the side of being too rich since erring on the other side can lead to catastrophic engine failure by premature detonation.

Now I’ll make you a suggestion for a terrific new engine oil that’s about to hit the market: Quaker State “Defy”. Many guys with high-output engines have flat tappets and highly loaded valve trains. These engines have a real appetite for oils that contain large amounts of ZDDP. ZDDP is the key ingredient used to reduce wear and it works by decomposing under heat and friction to produce a sacrificial layer in the high-stress contact zone. Unfortunately the “P” in ZDDP is phosphorus and it has been shown to have a negative effect on catalytic converters, and automakers restrict the amount that can be used in modern cars because the have to warrant the performance of catalytic converters for the life of the vehicle. Defy is formulated with 50% additional ZDDP and synthetic base stocks, and is targeted for vehicles with either higher mileage or high-performance engines. It is also specially formulated to reduce oil leaks past seals and was proven to improve compression. This oil should be available in early 2012 as was announced at the recent SEMA show in Las Vegas.