I have tried several additives... . Amsoil, Red Line, Howes (both forumulas), and Power Service.
I can't attribute ANYTHING, except a quieter running engine, which they all provided. I'm therefore "convinced" that the additives are providing some measure of lubrication.
So, I have settled on Power Service Diesel Kleen, a formula that does not provide anti-gelling, but supposedly has added cetane. Gelling is not an issue for me in my area climate.
The main reason I'm using Power Service is cost. I pay $9. 99 for a 96oz. jug. The "Kleen" formula is in a silver jug, while the regular formula is in a white jug, which is the one most readily available. The Kleen formula is usually not on the shelf, but my local NAPA store orders 4 jugs at a time for me.
I add 8 oz. every fill up, which is usually ~22/25 gallons.
Edit: I have been very interested in the Amalgamated product, and have watched posts from those using it, but I'm disappointed that no one seems to be stepping up with data that would convince me to order a 5 gallon pail of this stuff. I think illflem is right... .