Filling the MGB transmission with oil on the earlier 4 syncho gearboxes is done through a hole on top of the transmission. This is accessed from inside the car by removing a plug and then a dipstick. Of course, adding oil from inside the car in a tight space is challenging.
Approximately 2.5 Litres of 20W-50W oil is required. I decided the easiest way to accomplish the task was to set up an IV. I found a funnel that would closely fit the only piece of vinyl hose I had lying around. After starting the process of adding oil I realized I should have waited until I could purchase a larger hose. The 1/4 inch hose filled with thick oil slowly works its way down to the transmission. Very slowly! I haven’t timed it but I’m guessing that the whole process will take about 4 hours.
I’m using Castrol GTX 20W-50W motor oil in the transmission. Apparently this should not be used in an overdrive transmission so if you’re filling one of those look into it. Apparently many folks recommend gear oil for the overdrive.