This one I know!!
Hey how are you doing??
Toanswer you question, usually at start up, which is when there is the most vacumn. Some times when you let off the gas.
It will look like a puff of blue smoke.
Which is why when you re-do heads, it's very smart to re-do the seals. So E=me when you can..
How many mileage one motor? You can also check the color of your plugs to geet an idea of how bad or good it is.
Normal color I wouldn't worry.... HTH'ed