Engine oil usually appears either black if the oil is old, or a clear amber if the engine oil is new. Then there are a bunch of shades in between. Engine oil usually has a slippery feel when you rub it between your fingers. Sometimes it can have a distinctive odor, like old coffee. Most people know engine oil when they see it. If you’re in doubt, you could remove the dipstick from your engine and compare the fluid on the ground with what you see on the dipstick. If they match, it’s probably engine oil that’s leaking.
Engine oil normally leaks from the engine. However, it can leak from other places depending on the vehicle. Some vehicles use a separate cooler for the engine oil. If that’s the case, the oil might route to a different location away from the engine itself. Some oil filters are located away from the engine as well. If you have one of these systems, be sure to check the plumbing going to these external filters or coolers when looking for your engine oil leak.
Also, in the case of a turbo-charged vehicle, there are oil supply lines running to the turbo. I realize that the turbo is often located in the engine bay, but if you’re looking for a hard-to-find oil leak, it pays to know all the possible places that oil can leak from or into. If the seals go bad inside the turbo, oil can leak into it, where it will be burned. So if you have an oil leak you can’t find on a turbo-charged engine, be sure to include an inspection of the turbo. The turbo is one of those places oil can leak into and out of, so be sure to include it in your list of things to check when looking for oil leaks.
When searching for an oil leak, I start high and work my way down. Oil leaks travel, usually aided by gravity. You might be surprised at how far an oil leak will travel. I’ve seen oil leaks end up by the rear muffler. So if you discover your engine is leaking oil, start looking toward the top of the engine for the source. Valve covers in particular are known to leak from time to time. Since the leak is near the top of the engine, it will then travel along the outside of the engine on its way to the ground. This traveling can mask its true origin. So be sure to start high on the upper parts of the engine when looking for the source of your oil leak.