Should I flush the tranny first before filling it with synthetic oil?

What is your question?

My Ford is a Bronco II not a Ranger, but both have the same running gear. I normally use synthetic oil in the engine and running gear, but a while back I add the tranny checked because of a a slight whining noise. The mechanic check the oil level, but instead of topping the oil off he drained the tranny and replaced the oil with non-synthetic 80-90. I'm going to drain the tranny and replace the oil with Red Line 58304 / 75-90, and replace the transfer case ATF with Red Line 30504 D4 ATF. Should I flush the tranny of the non-synthetic 80-90 gear oil before filling it with Red Line? If so what should I use?