High thee thence to thy nearest auto parts store
and you will find a gadget which looks just like a traditional cartridge grease gun, except that it has no side handle as used to pump grease, but only the t-handle at the bottom, and instead of a grease tube with fitting adaptor at the top, there will just be a plastic tube, often transparent.
You stick the top tube in your can of gear lube (or whatever you mean to transfer from one container to another) and then pull the bottom handle out gradually, which fills the body of the tool with the lube. Then you remove the tube from the can and discover that you are spilling lube all over the floor or driveway from what is dripping off the tube and you grab a rag and wipe it off. Then you go to your differential or transmission (or whatever), which presumably you already had unplugged or else you're in for some more spillage while you set down the tool to get a wrench, and stick the tube into the filler hole (get your mind out of the gutter!) and then push the handle in to inject the lube into the differential. A few seconds later, you will be rushing to wipe up more spillage as the overfill comes rushing back out the filler hole (this delay must be some demonic trap!).
Then you draw off the excess with the same tool, plug up the gearbox or differential, squirt the excess back into the container, providing you didn't let the slightest bit of dirt get on the tool, and clean up everything, which may take a while the first few times until you get the hang of it.
Be sure that you always keep the tube on the tool spotlessly clean, inside and out, or you will be injecting grime into your rear end -- I mean, differential. It is especially frustrating to be under the car, almost ready to squirt the lube in, and then have the tube hit the bottom of the car chassis in some grimy place as you move around toward the target -- the only reasonable thing at this point is to crawl back out and clean everything again.
There are, however, more expensive tools which make some of this a bit easier. Should I have said that first?