Judging by the multitude of threads and posts, does not seem to be a straight forward answer. I will apologize up front for the long post.
I just joined up yesterday and am hoping to get an answer to this.
I have a 1973 Charger with a 69 440 and a 727 transmission that has not run or been driven since 1996. I am in the process of collecting parts, so I can get it back on the road and driving again, Phase 1 of the project.
One of the questions I had is what type of transmission fluid to use. In 1993 I pulled the transmission out and had a mechanic from work tear it apart and do the work I did not have tools for. I swapped in a clutch housing that was from a tranny that was behind a hemi, so could take 5 clutch discs instead of 4 (memory may be failing me, so please let me know if I am incorrect). Additionally I put in a Turbo Action shift kit and a 5.0 kickdown. I also had an additional 3 springs added (can't remember what the part is called) bringing it from 9 springs up to 12. My goal was to get a firmer and crisper shift.
I have done several looks through quite a few threads on the fluid type to use and it seems like there are several choices. I figured out that back in 1993, I put in Dextron III / Mercon. I was at Firestone yesterday and asked the manager there and he said I needed to flush out the transmission and put in ATF +4. His reasoning was that his brother had put in Dextron in his Charger transmission and it ended up burning it up. At the time I did not think about the fact that his brother's car is a newer Charger, so may be irrelevant to the discussion here, but it did scare me until I started doing some looking again.
I have seen a number of different answers on here and am still not 100% clear what direction I need to go. I will probably be draining the Transmission anyway, so there will not be a better type to switch types if that is what I want to do. I think I have seen a number of posts that say you can stay with the Dextron III and that it is a little more slippery than the Type F and the ATF +4. I also saw someone say that you can mix the Dextron III and Type F to whatever blend you want to firm up your shift however much you want. Others have said that ATF +4 is great and that is the way to go. Would I be correct in assuming that the Type F has a firmer shift and is less slippery than the ATF +4? At this point I am thinking I will probably move away from the Dextron III, but not sure between these other two. Also, need a little firmer answer on whether these can be mixed. I would probably drain the tranny, fill up the new Torque Converter and put it in and then fill up with the new fluid, so assuming there will be some of the old stuff in there yet if I do not flush it.
I will probably try calling a transmission shop locally as well, but hoping there will be a definitive answer from someone on this forum.
Thank you!
Allan