Originally Posted by redgoober4life
I think this was brought about because people were using regular one change, synthetic, regular, and changing back and forth. The gaskets expand and shrink a little each time, weakening them. If you change once, then you'll be ok.
Of course, if you have 150k miles, I really would question why you would want to change anyway.
That's pretty much the story I got. Why someone would switch back and forth like that is beyond me though.