i have an 84 ford 4x4 351w c6 and its about time i got around to servicing the differentials. Heres my question what weight of oil goes in them, im not sure of the gearing. Any help is greatly appreciated.

 

I would think 80w90 gear oil for both. If you have a limited slip diff you will also need the additive.

 

what additive will i need exactly

 

Only if you have a limited slip diff will you need the limited slip additive. You can get the gear oil with the additive already in it or you can get it seperately and add it to the axle before you put the gear oil in.
Ask at the store for limited slip diff additive or look on the gear oil container for "contains limited slip sdditive" or contains "friction modifier" and you should be OK.

 

okay will do thank you very much for your help