I have just changed the fluid on my 78 280 SLC with the 4 sp G76 manual transmission. I used Amoco Synthetic Mercon III ATF and the shifting is much smother now. I hope the "DOC" will be offering the transmission bushings for sale soon; I have been holding up on my parts orders till the store is up!
Still searching for a 300 E 5sp manual for my collection!