Ive heard things about places and oil, plugs being left out, engines being way over filled.
cracked or warped heads that fill the pan with that much coolant I doubt the engine will run at all. magnums are famous for cracking heads, mention them to a machinist and watch him flinch.
You generally don't get coolant in the oil, you get oil in the coolant because the oil system is at a higher pressure level.
hm, what was the temperature level when it started to sputter? and find out why they flushed the radiator. What does the inside of the radiator look like? whats the coolant look like? what does the oil look like?