It means that oil is getting into the cooling system. Most probable cause-bad head gasket or crack in the water journals somewhere in the engine. A compression test will give the mechanic an idea of the area of the problem.
A couple of ways. 1. Someone put the wrong fluid in the wrong place. But since you wouldn't do that I would guess that the engine block has a crack in it. The water jacket has been compromised and the fluids changed place. Check your oil and see if there is a milky substance on the bottom of the dipstick. If there is, well sorry, seek out a qualified mechanic.
The remains of the old coolant can cause the new coolant to fail. Using the wrong type of coolant can also damage the engine.
It can be a manifold gasket leak. These are some-times very hard to detect due to the high pressure the coolant turns into a mist while you are driving and seems to disappear. Also you might experience white smoke coming out of the tail pipe when first started and maybe continue for a few seconds. You can fix this 2 ways either with a $7-10 gasket repair or block seal, or do it the guaranteed way to fix it by replacing the gasket.
Sounds like a blown head gasket. Have a mechanic do a compression test, and check for oil in the coolant and/or coolant in the oil, (white foamy material around top of fluid level.)
Obviously it is leaking coolant. The real question is from where. Could have a dry-rotted coolant hose, a leaky radiator, or a blown head gasket. The first being the easiest and cheapest to fix and the latter the more expensive. If there is coolant around the bottom of your engine or in the driveway then its a leaky hose or radiator. If you have white smoke coming out of the exhaust or the oil looks milky on the dipstick, its got a blown head gasket and its burning the coolant off. Either way I would stay away until you know more about the problem.
Wrong mixture make sure you have pre diluted/mixed coolant/antifreeze.
Coolant is getting into the combustion chamber. Sounds like a head gasket problem.
your car may be leaking coolant into the engine, i.e your head gasket may be bad
No.
Well, the Milky Way doesn't only include stars but it includes planets too.
If only adding coolant to a leaking system it is safe because soon you will be forced to drain the coolant and fix a problem
sounds like a blown gasket, allowing coolant to enter the cylinders.