I use the small oil fill funnel that came with our generator. Got 3/4 of a gallon in a jug last night out of the big gear box, then set it up with a 1 lb coffee can (remember those lol) and the funnel and will let it drip all day. Also pulled the gear box drive pulley and belts to make room. When it's coming out fast, I have to use my finger to limit the flow since the funnel is pretty small. For the lower gear box, I'll probably bend a small piece of .030 aluminum scrap to channel the flow.
I'll be cleaning the big gear box tonight - stuck my finger down there and it came back looking like the trans pan on the race car after blowing out the high gear clutches in the C4.