Hi, I'm inclined to allow the pump to pump it out and replace the fluid in the reservoir as is goes to prevent too much cavitation, continue until you have a clean clear fluid being pumped out and that should flush the system enough not to be concerned about the change over of fluid type, most mineral fluids will go down below -20°C or so a synthetic fluid is most likely not required and the current fluid was working ok before it become contaminated, check the breather cap and see if there is any filtration as in most instances there is not and we usually pop a good quality plastic bag over the reservoir and cable tyre tightly so air tight with enough air space for expansion and contraction so you have created a “lung” type breather, that way as the system heats the bag will lightly expand and as the system cools the bag collapses preventing any environmental dirt entry and fluid life usual becomes years without dirt been able to gain entry, Regards Rob S