I have a 92 Ranger 3.0 with a 5 speed manual transmission. Had the rear main seal and clutch replaced and when i got the truck back the mechanic had replaced the ATF in the transmission with 10W-40 Transmission oil. How do i flush out all of the oil and get ATF back in the transmission without being contaminated by the transmission oil. When i asked him why he had put transmission oil in he told me that ATF breaks down the freeze plugs at the top of the transmission and leaks.

 

Welcome to the forum

Just drain it and put back in Mercon V, or synthetic equivalent
Remaining oil won't hurt anything

If only holds 2.8quarts, so most refills only take a little over 2 quarts

ATF is about 5w10-5w20 if it was measured in that way

I would drive it for 10-15min shifting thru all the gear to get current oil heated up
Then open the drain plug to drain it
You can also remove fill plug first for faster draining, trans is vented at the top but not a big hole to let air in
Drain and fill are 15/16" heads

The rubber shift rail plugs at the top rear of shifter housing can dry out and leak, but not because of the oil used internally, its more on where you live, hot and dry climate causes all the rubber parts in a vehicle to dry out more, lol