When I searched for fuel additive bottles, many companies only sold HDPE 2 bottles in bulk quantities. For example, Consolidated Plastics only sells bottles by the dozen.
I can't post a direct link to any sales page on BITOG, but you can go to the US Plastic website, then SKU Search for 66100 and 66419 (32 oz bottle and 28/410 Yorker Spout Cap). The bottle has graduations every 2 ounces with a 1/2 or 1 oz dispensing chamber. The Yorker Spout is far easier to keep clean than open threads with a big cap.
I've used this bottle for three fills, and the plastic has shown no signs of cracking or splitting. My only complaint is the bottle's oval shape needs a two-hand hold. Keep the spout on top (see their picture), or it won't measure right.
