Doug,
Just a note that I have moved your thread and conversation to the Driveline forum as the best place to further discuss this topic.
In respect to your question about what weight oil should be used in an SM420 transmission, that has been discussed quite often. You could use the
Search The Site tool found in the left margin to see many of the threads on the topic that already exist.
Otherwise, have you looked in any of the easily found sources like original/reprinted and even on-line copies of owners manuals and Service Manuals???
See below for a screen shot of an "Owners Manual" page concerning transmission lubricants to be used (Note: this was taken from a '48 Chevy Truck Owners Manual, but is the same for your '49).
I also wonder why you asked about using an "additive"???
Hope this helps.
Dan
