How to Know When to Change Your Oil: Degradation on the Molecular Level

A car's oil should be changed between every 3,000 to 4,000 miles.

With all the hype surrounding oil changes, it's difficult to cut through the advertising down to the basic science of an oil change. Well, I'm not affiliated with Castrol, Mobil, Pennzoil, Motorcraft, LucasOil, or any other company that markets petroleum-based products, so I can honestly and fairly evaluate my experiences with oil changes. Since I began driving, I have never utilized someone else's services to change my oil, so I know exactly what brands and what quantity of oil has gone into my engine with each and every change. I have charted exactly how many miles it has been since each oil change (tonight, 25 June 2010, for example, I let it go a little farther than normal at 3,968 miles). I have also marked down the brand and viscosity used with each change and charted the performance of my car by date and corresponding with the correct oil change. I do this all in a vehicle notebook specifically tailored for this purpose. In short, my Honda is a rolling scientific research experiment.

On August 18, 2009, I was ecstatic to receive my current daily driver from my fiancée’s father: a 1991 Honda Accord LX (5 speed manual 2.2 liter 4 cylinder engine). It was a completely average car, but you wouldn't know it from the gigantic smile on my face every time I'm in the driver's seat! Before that point, I drove a hulking 2001 Explorer XLT (automatic V8 5.0 liter). The Explorer is a solid, reliable vehicle, but the gas mileage (19 on an average day) began to eat into my wallet. Therefore, I chose to drop the insurance on the Explorer and change to the Accord, which had 258,000 miles on the dashboard when I received it.

The oil in the car at the time was standard motor oil (not synthetic), but I'm unsure of the brand. My fiancée’s father charted the fuel efficiency of the vehicle about as closely as I do, and he told me that it got 30 miles per gallon like clockwork. While I was driving on that oil, I got the exact same mileage (from 29.8 to 30.5 mpg). The first few oil changes, I used Motorcraft standard motor oil (10w30 and then 5w30 once I ensured it did not leak). My mileage remained at an even 30 mpg.

In September 2009, I started using a 5w30 Pennzoil synthetic blend (50% motor oil, 50% synthetic, and my mileage immediately went up about 1.5 miles per gallon to just under 32 average. In November, I began using 5w30 Mobil Synthetic oil, and my mileage increased slowly over the next month from 32 to between 34 and 36 miles per gallon, where it has remained since even during the summer months. I have continued to use Mobil since then (even though it is the priciest oil).

I noticed that neither the synthetic blend nor pure synthetic resulted in any degradation in gas mileage over the first 3,500-4,000 miles (I never went further than that on the same oil and typically changed it at around 3,000). I also noticed that the oil still retained much of its original honey color at 3,000 miles. Regular motor oil, however, resulted in mpg that consistently began to degrade around 2,000 miles. I would start with 30.5 or so and end up with 29.5 by the last couple tanks. The color also blackened much quicker.

Gas mileage is the only tool generating hard numbers that I have to evaluate the performance of the oil I used. I have no ability to measure the lubricating quality of the oil, nor do I own a dyno with which I can test horsepower. I will say that when I made the jump from synthetic blend to full synthetic, I swore that I could feel an increase in engine response. Obviously, though, feelings are not scientifically measurable.

What I can say for sure is the following: my gas mileage was at 30 mpg with Motorcraft motor oil, 31.5 with Pennzoil synthetic blend, and 35 and even 35.8 once with full synthetic.

Confounding factor: I was learning to drive a new vehicle with a stick shift; this can account for some variable of change.

Clarifying factor: I drove on highways and cities at about 50/50 each; my mileage, therefore, would not be significantly altered by driving conditions. I also always drove alone during this period.

I noticed that 3,000 oil seemed to be right about the right time to change the motor oil. Repeated 3,000 mile changes with synthetic, however, have resulted in oil that is still similar to the way it came out of the bottle. Tonight, just under 4,000 was too much: the oil was slightly black. If you want to change your oil in a risk-free window, then, stick to changing it every 3,000 to 3,500 miles.

Note: on every synthetic jug I have bought, their is some claim about it being guaranteed for 15,000 miles. Heaven help you if you try to milk that promise for what it's worth, and your engine will suffer the consequences (that, I can guarantee). No kind of oil should ever be taken 15,000 miles. Even if synthetic oil can retain its lubricating properties that long (open for debate), it will be so full of impurities that that ability will be irrelevant. Change it less often than 4,000 miles at your own risk.

Experience: 6 years of driving, details below.

1. 5 years driving with the 2001 Ford Explorer, now with 140,000 miles. No breaking down, no engine problems, no rough running, no overheating, no nothing.

2. 1 year of driving with the 1991 Honda Accord, now with 279,000 miles. Also, no breaking down, no engine problems, no rough running, no overheating, no nothing.


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Posted on Jun 26, 2010