I guess much depends on what is available locally, cost per gallon, and how cold the temps are. From all real tests I've read, two-stroke oil easily beats B20 when it comes to cost per tank. But, I have no idea what you pay for B20.According to the Diesel Place test the most lubrious fuel additive is by far 20% Bio Diesel (B20), which I am glad to say is sold around here by Shell, there are SOME benefits to living in hippyland...
Stanadyne calls for fuel with lube that permits a metal wear scar at no more then 460 microns. That's what #2 diesel was before the low-sulfur stuff took over. Now it can be as high at 600 mircons of wear.
New ultra-low sulfur pump fuel has lube additives put in before you buty, and can allow 300 - 600 microns in that wear scar test.
B20 "biodiesel" is usually tested at 520 microns for wear. Of course, that might vary - since I've only read test reports of a few brands.
B100 - i.e. 100% bio-diesel usually tests at 300 microns of wear.
#2 ultra-low diesel with two stroke oil added at a 1 to 200 ratio - yields fuel that tests at 138 to 438 microns of wear. Cost around $1.50 to treat a 28 gallon fuel tank.