To find out what happens when you forget to change your oil, I sent a 12,000 mile oil sample to a laboratory. The results are not good, and should serve as ammunition to convince your stubborn friend or family member that no, it’s not “going to be fine for another few months.”

Earlier this winter, my brother and I were talking about his car at our annual family Christmas gathering. “How’s that crap-can doing these days?”, I joked about his Sunburst Orange 2007 Dodge Caliber, which inexplicably has almost 150,000 miles on it.

“It’s been making these weird noises lately. I should probably change the oil at some point,” he responded with a smirk on his face.

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He then admitted that he hadn’t change his engine’s lifeblood—the fluid that lubricates, cools, cleans and prevents corrosion—in more than 10,000 miles. Upon hearing this, I did a spit take and scolded him for his terrible, terrible life choices, reminding him that he’s sending his car to an early grave.

Okay, so I didn’t actually scold him, perhaps because a hopelessly terrible car like a Dodge Caliber going to a scrapyard might actually be a good thing for car culture, and maybe because my brother could easily beat me up. Really though, I understood that my brother knew not the implications of his inaction.

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The much-repeated adage is that you have to get your oil changed every 3,000 miles or three months. On modern cars, with synthetic oil, you can go a lot higher. On my cars I stick to 3,000; on my brother’s Caliber, 5,000 is probably safe. But 12,000? He was playing with fire.

To make my brother understand why, I sent a sample of his oil to Blackstone Laboratories in Fort Wayne, Indiana. And boy, were the results shocking.

So how were the results? Not good! His lab results show 0.5 percent insoluble content, which is within the recommended range, but a representative from Blackstone Labs told me most engines show no more than 0.2 percent, so this is cause for concern.

In the end, the results show high levels of iron in the oil, indicating wear of steel parts like the camshaft and cylinder liners. The universal average amount of iron found in that engine, based on an average 5,400 mile oil change, is 11 parts per million.

My brother’s result: 167. That’s more than 15 times as much iron in the oil!

Blackstone Labs told me engines can withstand high iron wear much better than aluminum or chromium wear. And on those fronts, my brother’s Caliber fares no better. His aluminum and chromium values are both five times the universal averages, indicating piston and piston ring wear, respectively.

If you want to learn more about what each row of the lab report means, Blackstone’s got an explainer on their website. It’s pretty interesting to see what a simple oil analysis can tell you about your car.

See, my brother didn’t know that, as oil is subject to high temperatures and as anti-oxidizing and anti-wear additives break down, sludge can form, making it difficult for oil to flow and squeeze itself between moving metal parts. Or that his viscosity modifiers break down after exposure to high temperatures, making the oil thinner at high temps and less effective at separating metal parts sliding past one another.

My brother didn’t know that metal, dirt, soot and other particulate buildup in his oil—much of which is too small to get caught by the filter—causes the fluid to become abrasive, significantly accelerating engine wear.

He didn’t know he could prevent this increased wear by flushing those abrasive particles out via an oil change. My brother didn’t know that, over time, detergents and dispersants meant to combat those small insolubles break down, and are unable to protect the engine.

He also probably didn’t know he likely has something wrong with his air intake, because the lab results show very high levels of silicon (three times the universal average), indicating that dirt and dust is making it past the filter.

My brother’s report was all bad news, so the takeaway here is: change your damn oil, people! You don’t have to change it every 3,000 miles. In fact, on some newer engines using synthetic oil, you could probably get away with a 12,000 mile oil change, but I think that’s probably pushing it. Check your service manual, make sure you know your recommended oil change interval, and save your car from oblivion!

Whatever you do, just realize how important that amber fluid is in your engine. It’s what keeps your car’s heart beating.

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