Absolutely. To not tell the owner is not doing your job. But you also have to know about it. You can't expect a mechanic to know about the oil unless he has a reason to look in the radiator. That oil may not make it into the reservoir where it would be seen during other routine service such as an oil change.
I don't know what you mean by "miss the oil". Do you mean would we notice the engine oil is low on the dipstick? No. As long as the level is between "minimum" and "maximum", we would have no cause for concern.
The more important question is where did the oil come from? If it's engine oil, it most likely came from a leaking cylinder head gasket. The oil will rot the heater and radiator hoses over time. If coolant is also getting into the engine oil, that will melt the soft first layer of the metal engine bearings and lead to an expensive failure. More commonly, if you have an automatic transmission, the fluid cooler inside the radiator is likely rusted out. Coolant mixing into the transmission is also not good because among other things it doesn't have the lubricating properties needed.
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Monday, February 25th, 2013 AT 11:13 PM