Well thicker juice takes a little bit more adjustment to fill things up. I pull out the cart filling, fill cart about 2/3's of the way, then use tweezers to push the filling back in slowly. A syringe and needle would work good, it's easy to buy a meat tenderizing kit which has a big hollow needle at the grocery store. Here in NY state good luck getting needles unless you have a great relationship with your veterinarian!
Also, when using thicker juice the stock poly filling in an 801 cart will be too dense to allow for easy wicking. Pull it out, rinse it, then cut about a third off and fluff the rest up a bit to fill the same volume but more loosely. That will wick the thicker VG better.
I don't thin this stuff at all, just watch the wicking and make sure things stay nice and wet.