Keep them in separate bottles. If it's in your car engine, you need serious repairs.
Change the head gasket.
short answer, your getting water in the oil.
if the timing chain and tensioners are worn out the chain will wear a hole in the timing cover and the oil will get into the water jacket and the antifreeze will get into the oil high time to fix it
Possibly blown head gasket. Very serious condition. Have this repaired ASAP, and stop driving the car until you do.
To stop water from getting in unwanted places
By refrigerating it.
if water is getting in the oil you have a blown head gasket take it to a mechanic if you are just getting condensation which will look like oil and water sludge on inside of radiator cap, there is not much that can be done about that
Getting In - 2010 Oil and Water 1-7 was released on: USA: 28 April 2012
Apply a non-oil sun block
if you are/have in the past used synthetic oil stop. it will take two or three oil chances to stop this. if you run the motor six to eight hours in summer this will stop this too. the cause is water condensing in the oil compartment
I'm guessing nothing the oil just sits on top of the water and chemicals are used to make the oil dissolves. Like BP oil spill
rx8 light does not come on when water and oil mix. if the oil and water mix is up the dipstick then this is norm more so on short journeys