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

(1990 Ford Ranger)
in Prairie City, OR on March 03, 2010
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?
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Popular Answer
on March 03, 2010
Just drain and reinstall the Red Line because too many things might go wrong with any type of flushing chemical
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on September 28, 2010
well its gunna have the old fluid in the cooler too so if ur gunna put synthetic in it u gotta flush it all
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