I picked up a 2005 blazer a( 3 button 4×4)and I am working through a to-do list to bring her back to life. I went to the GM dealership and picked up the fluids to do the front and rear diff. They told me the do not have the ac Delco 80w/90 and that it has been replaced by the synthetic version 75/90. Should I do a flush since I am going from regular gear oil to synthetic or does it matter? Is there any seal compatability issues? If I decide to flush, what is the best stuff to use and the best method? Thank you.