(Spigga @ Oct. 09 2006,12:51) Anyone who tells you that believes this. Some synthetic residue will stay on the engine parts after the enigne is drained. Once you put the regular oil in it will slide off of the synthetic oil that was left in the engine, prohibiting the regular oil from doing its job properly.
I was told the "once synthetic always synthetic" thing a few years back. When I asked why, that was the explanation I was given.