blowing smoke?

[big green91]

i have a 91 explorer with a 4.0 v6 it has 168,000 miles on it,but it still runs great thanks to a little tlc my question is that every once in a while when i get on the gas from a stop sighn or to pass someone i get a puff of blue smoke for a sec or two out the tail pipe.now i know that the blue smoke means that im burning oil but can someone please tell me whats causeing it and why it only does this once in a while and is there anything i can do to it my self besides takeing it to a garage only to get ripped off.im sorry for such a stupid question i havent owned this explorer that long and i dont know that much about fixing engines any help would be greatly appreciated thanks again
scott
ne pennsylvania

 

[Crankcase]

How much oil does it use up?

 

[big green91]

i will fill it up with oil and it will run down about a quart or two of oil in about a week or so.also i use royal purple synthetic 15w40 in case that matters it never leaks any though i have put cardbord under it when i park it at home and there is never a drop on it i also change my oil anf filter every 3000 miles

 

[aldive]

Probably bad piston rings, Run a compression test.