If you can't find a syringe that fits the bill, there's one hack you could use. Let's say you're trying to inject .1 mL of E medicated oil.

First, obtain a vial of sterile oil of some sort. You want a sterile vial with an injection port that contains sterile oil inside. (Ideally, this would be the same oil as whatever is used as the carrier oil for your medication.)

When giving yourself a shot, first draw .5 mL of the plain sterile oil. Next, draw .1 mL of your E medication. You now have a syringe that has .1 mL of E oil sitting on top of .5 mL of sterile filler oil.

Inject yourself with the syringe. The E oil will be entirely injected, and all the dead space inside the syringe and needle will be filled with the dummy oil. You'll inject yourself with the full amount of E oil and a little bit of dummy oil.

I don't see any reason why this wouldn't work. Finding sterile oil vials could be tricky, but it's probably doable. Sterile vials of bacteriostatic water are a lot easier to find, but I'm not sure if that would cause problems with the differential density between the oil and the water. The oil would want to float to the top of the column (though this may not occur in the time it takes to draw and inject.) While this would be fine with drawing (oil on top, water on bottom), when you invert the syringe to inject downwards, the oil and water would want to switch places. You could try to inject yourself with the needle pointed upward, but that would be a bit of a feat of contortion. Though, if you're injecting into the top surface of your thigh, I suppose you could lay stomach side down between two chairs, with your thigh exposed in the gap in between. It would be a minor feat of contortion, but it wouldn't be too hard.

I would suggest trying to find vials of sterile oil. If you can't, sacrifice a syringe to science and experiment. Try drawing some ordinary tap water and some olive or other oil into a syringe. First draw some tap water into the syringe, then draw some olive oil. See if the system is viscous enough that the oil will stay on the needle end of the syringe even if inverted. If this works, then you should be able to do this dual layer technique even with more easily obtainable vials of bacteriostatic water. Obviously don't inject yourself with this experimental syringe.

Some dead space in your needles is inevitable. But there's nothing that says that your actual medication needs to occupy that dead space.

EDIT: disregard this. Try the airlock method instead. https://old.reddit.com/r/DrWillPowers/comments/yv37fy/estradiol_dropped_by_200_pgml_after_switching_to/iweyuva/?context=3