JYF,, Glad I come and check on where you guys are hiding out once in a while. WOW!! When they say "different strokes for different folks"- I now know what they mean. There are hundreds of ways to "shade tree mechanic" fixes on these old I6 235s but that is all they are. I will admit that I am probably the laziest guy you ever saw but the other thing is- I just plain have to "do it right" and do it ONLY once. I wouldn't advise nor suggest using ATF in an old 235, especially if it is something you wish to keep. If you are wanting to pretty it up for a potential buyer, then it will make your oil "look good" but sure isn't gonna help the engine much. Same goes for using Diesel, it is only a "shade tree" trick. These 235s are really simple engines and if you follow some simple rules working on them is easy. When I rebuild an engine I always advise changing the oil every 1,500 miles. Most would say you don't need to do it that often, but it doesn't allow impurities to build up and usually your oil is old anyways because you aren't getting to many miles in a short time. Don't use detergent oil in an old 235, (unless it is a newly rebuilt) and when you use ATF or diesel be aware that you may dislodge deposits that could plug a vital oil line and then give you really big problems.
Now, as to your rear main seal leak. There is a "tech tip" that I wrote that is really "Blonde" proof. If you run into any problems or confusion with it just email me and I will assist you.
Rear main seal replace
