If you are seeing oil on the cooler lines replace them ASAP. Could also be where your oil is going. As the car heats up the thermostat opens and lets oil flow to the cooler under very high pressure. You will not see much of a leak if any at all at idle or when the car is cold. I had a tiny hole open up a couple of years ago on a trip back from KY and lost 4 qts in 300 miles of highway driving.