OK guys...
I am going to tell you that the once and for all, end of discussion, solve all of our problems answer does not exist for this topic. I was one of the guts in the shop back when the change from gear oil to atf was coming about in American vehicles.
We searched for years to find the ryhme or reason for changing one vehicle and not the other. We even compared internals to find a dis-similar part between transmissions that would be the weak link calling for one or the other.
We never got a straight answer from the auto manufacturers, borg-warner, or any of the parts suppliers.
The only "answer" that has keep itself true was that there was a lubrication problem with the gear oil and so the switch to atf was made. My hypothesis is that the engineers that designed this trans, (origionally for the s-10 by the way) made some incorrect assumptions regarding the fluid characteristics of the gear lube. At this point the plan to change was made (to atf to easily tell it apart from gear oil for warranty purposes) but some applications were held from the change for some concern of applications that would load the bearing surfaces to a larger extent. However, this last part is where there is not much ryhme or reason. One application gets atf and a different one gets gear oil. Some engineer asked to get some models changed but did not ask for all applications to change.
Also, internal parts can not be interchanged between brand as mentioned above. The gear sets have different cuts even between the same ratios on many applications. Yes, you can remove all of the internals from one to another. Yes, you can take all of the gears and parts that make up, say, first gear on a Ford t-5 wc and put them in a gm t-5 wc to change the ratio, but that is almost the extent of it. You can not haphazardly take parts from any wc t-5 and put them in another wc t-5, nor can you take nwc gear parts and use them in a wc. That said, there are SOME internal arts that can be swapped and others that can be "creatively swapped."
Not venting-Just wanted this in the thread for future readers that want to do the swap.
