I'll tell you what I do and then sit back and watch everyone tell me that I do it wrong.
First, my
oil cooler is on the other side of a thermostat. The only way that I'd be able to get the
oil out of the cooler and lines is to disconnect everything. I don't bother.
So, what I do is drain the pan on my 351W (both forward and rear drains), remove and replace the
oil filter. Drop in 5 qts of oil ... and next the tricky part: I turn on a PreLuber oil pump and bring the system up to pressure.
Finally, I measure the oil level on the dip stick (5 qts seems to be perfect) take the car for a spin to warm the engine up, measure again.