There should be a metal pipe plug on the middle section of the differential about half way up (front or back). you fill with 90 weight gear oil to the bottom of the hole. What is in there from the factory should last the life of the vehicle and you should not need to add unless you had a leak or unless work was done.
1.5 qts of 80w90 oil 1.5 qtys of 80w90 weight oil
The rear axle.
About 1.25 liters of oil goes in the rear axle of a Ford transit. It takes 75W90 oil when you replace it.
synthetic gear oil
It's the oil/fluid that is in the rear drive axle (and front axle on 4x4).
Axle seals
In many cases they are lubricated from the gear oil in the differential.
Remove rear axle cover, oil will drain out, reseal and install cover, fill fluid.Remove rear axle cover, oil will drain out, reseal and install cover, fill fluid.
purple
This depends on what Buick it is. Most do not have oil in the rear "Axle" as most Buicks of this vintage did not have a rear Differential. The ones that did need oil in the differential have a fill plug on the rear cover or the side of the case and this is where the oil is poured in. Follow the instructions in the owners manual to get the correct level.
There are no "grease" points on the rear axle. The gear oil is checked and filled through the plug in the differential cover.There are no "grease" points on the rear axle. The gear oil is checked and filled through the plug in the differential cover.
75w90