Dipstick oil analysis may sound a little goofy, but it works. Not to mention it’s also cheap and quick. There’s only one problem - figuring out what the oil on the dipstick really means. No worry. Sit tight for five easy lessons on reading your dipstick.

Lesson No. 1 - Retrieving the Dipstick

  • With the engine hot, park on level ground and shut off the engine. Wait a couple minutes for the oil to return to the oil pan.
  • Open the hood and find the dipstick on the engine - a metal loop or grip sticking out of the end of a metal stalk. If you can’t find it, your owner’s manual should help.
  • With a rag or thick paper towel in one hand, pull on the metal loop or grip and remove the dipstick with the other. Wipe the oil-wet straight end of the dipstick and push it back into the stalk you pulled it out of.
  • Wait a few seconds and pull out the dipstick again.

Lesson No. 2 - Detecting a Low Oil Level

  • Examine the end of the dipstick and notice where the oil ends. There are markings that indicate the level the oil should reach.
  • If the oil doesn’t reach inside the markings on the dipstick, you need to add at least one quart of oil. The amount of make-up oil you would expect to add will vary depending on the age of your car, type of engine, total mileage and driving conditions. The dipstick is your gauge for abnormally high oil consumption. Real concern begins at about one quart for every 1,000 miles (0.95 liters for every 1,600 kilometers). It’s time to plan an overhaul if the problem advances to one quart every 500 miles (800 kilometers).
  • Is it OK to be a quart low? The sidebar at the bottom of this article can answer this question.

Lesson No. 3 - What If the Oil Level is Too High?

  • In certain cases, the oil level may have risen since the last time you checked. This could be due to condensed water (from combustion), condensed fuel or a coolant leak - all are causes for concern.
  • Fuel-diluted motor oil (from blow-by or leakage) can substantially reduce oil viscosity and thin additive concentration. The odor of diesel fuel can often be detected right from the dipstick.
  • Free and emulsified water is harmful to the oil and the engine. For short-trip drivers, water condensation may be more acute if your engine has the flexible fuel vehicle (FFV) option and you are burning an alcohol-gasoline fuel blend. It is important to remember that combustion produces water in your engine - more water than the fuel consumed. Most of the water goes out the tailpipe, but if the engine is cool, much of it may condense in the crankcase.
  • A simple way to detect water in used motor oil is to put a drop of oil from the dipstick on a hot exhaust manifold. If it crackles (sounds like bacon frying) this is an indication of water contamination. Beware that there is some risk that the drop of oil may catch fire.
  • Brand new automobiles imported from Japan may have a high oil level due to short-run engine starts (as many as 50) required when the vehicle is transported from the assembly plant, across the ocean, and finally to the dealer’s lot. In this case, an oil and filter change may be merited.
  • Coolant leak is a serious problem relating to high oil level. See Lesson No. 5.
  • Whatever the cause of the high oil level, the condition needs to be quickly corrected.
  • Note: accidental overfilling oil into your engine can cause problems too. As the crankshaft rotates it will churn the oil, causing aeration and eventually sustained foam may form. This can lead to overheated motor oil, oxidation and a loss of oil pressure. Spongy aerated oil is hard to pump. It starves the engine and critical lubricated surfaces.

Lesson No. 4 - Signs of an Aged Oil

  • Oil is not like a fine wine that gets better over time. Instead, it ages at a rate that is influenced by driving conditions, fuel quality, engine age, motor oil quality and climate. If not changed in time, your oil will wither and fail to protect your engine.
  • So, let’s take a close look at the oil on the dipstick. The oil should look smooth and glossy and somewhat transparent. If it has sludgy deposits or grainy particles of dirt, it’s time for an oil change. The same is true if the oil looks too thick, is too dark (opaque), and/or has a putrid rotten-cheese smell.
  • If you still don’t know whether you need an oil change, consider doing a blotter spot test.
  • Oxidized and contaminated oil will lose interfacial tension. A simple test for interfacial tension is to place a drop of used oil from the dipstick on the surface of water. If the oil drop spreads out over the water’s surface (instead of beading up like a new oil) it may be time for an oil change.

Lesson No. 5 - Signs of Coolant Leak

  • Brown bubbles or a dried crusty-brown residue above the oil level line on the dipstick could be an indication that coolant (water and antifreeze) has leaked into your engine. The oil on the dipstick might even look like chocolate milk. Never taste motor oil as a test for antifreeze.
  • Another prominent indication of coolant leak is white exhaust smoke that has a sweet odor. In this case, the dipstick oil level may actually rise, indicating a significant amount of coolant has leaked into the crankcase.
  • To confirm a coolant leak, shut off the engine, let it set for an hour or two, unthread the drain plug and use a clear glass or plastic bottle to catch the liquid. Because both water and antifreeze are heavier than oil, they will puddle up at the bottom of the oil pan. Collect a couple of ounces of fluid and immediately retighten the drain. Inspect the fluid for glycol and water. Glycol and water often look like a thick mayonnaise-like paste, depending on how long the coolant has been in the crankcase. You might also detect a sweet antifreeze smell.
  • If you have detected coolant in your motor oil, your engine should be taken in for immediate service.

So there you have it - dipstick oil analysis in five simple lessons. Checking your oil level may never be the same.

Have you learned tricks, not mentioned here, for using your dipstick to analyze used motor oil? If so, please share these ideas but beware, they might show up in print someday!

Is it OK to Be a Quart Low?

Automakers and owner’s manuals will often say it’s OK to wait until the oil level falls below the add mark to add oil. However, remember the crankcase of most passenger cars today holds only about four quarts of oil. This means you are running the engine with 25 percent less oil (one quart), which may not be wise.

Motor oil has many important functions beyond just controlling friction and wear. Oil not only lubricates the engine’s internal parts, but also helps cool the bearings and other frictional surfaces. The oil in the engine, therefore, serves as a heat sink to gather up unwanted heat to transfer it by conduction or convection out of the engine. Under typical driving conditions, running a quart low may not make much difference in terms of bearing temperature or overall engine lubrication. However, the engine also needs to be protected under worst-case conditions, such as in hot weather, while towing or with an impaired cooling system.

Likewise, when you’re 25 percent low on oil, you are also 25 percent low on critical additives - the additives that prolong the oil life and the engine. Additionally, when you are 25 percent low on oil there is less oil to disperse harmful contaminants, acids, soot, fuel, sludge and water that enter the crankcase. When you do the math, a quart low on oil translates to a 33 percent increase in contaminant concentration. Plus, the remaining oil spends less time at rest in the oil pan and more time in the hot frictional zones of the engines. The added heat, shear and pressure will more rapidly degrade the oil and its additives.

Once all of this is brought into the picture, especially the prospect of an extended oil change, for many car owners it’s a good idea to add oil whenever the dipstick reads low. Don’t wait until it is down a full quart. If it needs half a quart, add half a quart to bring it back up to the full mark. However, be careful not to overfill the engine. In addition to the problems caused from overfilling the crankcase, too much oil may cause leaks as the high-riding oil is more easily forced past seals and gaskets.

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