To add to the confusion, the service manual for my '54 1/2 ton w/ torque tube and SM420 four speed says to remove the little plug on the torque tube ball joint and insert a "few"ounces of 90W gear lube AFTER rebuild. Because there is no rear seal in this transmission, it is assumed that gear oil from the tranny will leak into the torque tube ball joint and keep the u joint lubricated. There is a seal and a bushing around the internal driveshaft at the front of the torque tube that is designed to keep tranny oil from leaking past this seal and overfilling the differential. This bushing and oil seal can be removed with a special tool and replaced with a new integrated seal and bushing called an "Okie" bushing without disassembling the differential and removing the driveshaft.
Also, I find it easier to fill or top off my SM420 four speed by removing the gearshft and pouring gear oil into the top of the tranny until it runs out of the fill hole on the side of the tranny.
