modifry

^ This works well if you have extra fluid so you can keep the bottle full.

Of course I also have an over-engineered pressure feed system where I pour the fluid into a container, screw on a lid with a dip tube, and then pressurize it to 10-15psi. There is a ball valve in the outlet tube so it can be shut off as soon as the tranny/diff is full.

Car Analogy

I've done similar, but added a short piece of clear tubing stuck onto end of gear oil top nipple, since sometimes you can't get the bottle close enough to the fill hole. Just have to be careful not to squeeze bottle too vigorously, lest the tubing get pushed off and you make a big mess (helps if oil is warm so it doesn't create as much pressure).