I think that if it's losing as much as to be noticable (overflow tank?), it would manifest itself in more ways than a bit of froth in the oil filler cap. It would add to the oil level and be visible on the dipstick.

Do you notice any odor of hot antifreeze after driving? I had 2 cracked heads in the Ranger I just Cash-for-Clunkered, and would always lose ~ 2 liters/month and notice that smell.

No antifreeze in the oil, though.