Hi Neil

The diff oil is very specific and you must fill with correct oil, the level of oil is usually up to the plug on the side definately not fill it up to top...

Thye GoMog website is a good bible for any work on your morgan and lists the oils required.

see here

https://www.gomog.com/allmorgan/axlefilling.html#SALISBURY


Morgans 1934 MX, 1947 Series 1, 1956 +4 TR4, 2000 +4 T16
Triumph Herald 1969 13/60
Morris 1970