It is a volume and flow problem. 2% of a 1000 gallons (for example) is 20 gallons. If you dump 20 gallons of biodiesel into 50 gallons of residual and then have your supplier fill it the rest of the way with petroleum diesel, no problem. If you have 800 gallons in the tank, add 20 gallons of biodiesel and then have that topped off with another 150 gallons of petroleum diesel you are asking for trouble. In warm weather, trouble seldom develops but it can. In mixing fuels, a gentle hand is bad and a funnel with a long tapered spout is the worst. The funnel can easily give you laminar flow of the biodiesel, especially if you keep the funnel full as you empty your container. You can get biodiesel entering a tank with nary a splash. In a reasonably empty 1000 gallon tank, you’ll have a 4’ fuel drop between the fill port and the residual fuel. Lots of splashing because the stream breaks up which is what you want.

Most fueling pumps don’t pump fast enough (or run long enough, or pump at a high enough pressure) to do a good job of mixing. At 30 GPM, you have to run the pump for 33 minutes to pump 1000 gallons. Then, the suction line is typically 3” or 4” off of the bottom.

An inline static mixer solves the problem. Stick your fuel pump nozzle in the pipe leading to the inline mixer, turn it on and pour in the biodiesel. If you can pour 5 gallons per minute and the fuel nozzle pumps 30 GPM, you’ve solved your density problem.

You like to fabricate so lets conjure up a $5 solution that still lets you stick the tank. Assuming the tank uses a 2” bung. A foot of 2” pipe threaded on both ends, a 2”x2”x2” tee and a 90 degree 2” ell plus a plug to cap the ell when it isn’t being used. Move the existing 2" fill port to the top of the Tee. Then take a piece of ¼” rod, make a T handle so you can’t drop it in the tank and weld a couple of nuts to the long arm of the T (which is still shorter than the length of the pipe plus the tee) to provide the splash. Drop it in whenever you mix up a batch.

You approach will work if your have 20 plus gallons of petroleum diesel in there to start with (which changes your desnsity difference by 50%), the tank isn't well baffled and you have a heavy foot on the brake/throttle (or rough roads).

Yes, it is worth it. The lubricity from biodiesel (anything over 0.5%) is really quite amazing.