For all those with M6 transmission (gyromatic, tip-toe, unerdrive). While I was in my local NAPA dealer today I saw quarts of 10w non detergent oil. I asked the clerk if this was something they usually carried and he said no. I flipped the bottle over and found the website for the distributor.
www.smittysinc.net They are a wholesaler and probably won't sell a few bottles to an individual, but they should be able to tell you where you can find a retailer. I went to their website couldn't see any specific reference to the 10w, but I know it exists because I had it in my hand.