From the Morris Minor Owner's Manual, Section PP.2A
The gearbox oil level should be checked by the dipstick every 1,000 miles (1,600 km) and replenished if necessary.
The filler plug, which also serves to indicate the oil level, is located beneath a rubber cover situated near the gear lever, and is accessible when the front carpet and rubber cover have been raised.
The gearbox must only be filled with engine oil to Ref. A (SAE 30 grade) to the level of the filler plug.
After the first 500 miles, the gearbox should be drained and filled with fresh oil. This procedure should be repeated afterwards every 6,000 miles (10,000 km).