u dont want to mix the oils at the same time. it dosent matter if u got from full syn to standard at any time. they are made by law to be compatible these days. its not like the old days when u couldnt. if u switch all the time u dont get the benifits of the syn. there for u are just wasting money going back and forth.

as far as break down in oil the syn wont break down as fast as standard oil. the additive package will break down inside the syn oil faster then the oil will break down