I trust your mechanic.
Break down in terms 135Km and 3 ltr?
The rear diff, is noted in your owners manual time and time again we ignore the info that is right in front of us, if it was digitally printed I bet we pay more attention?
1 qt and 1 ounce is the required rear diff change, you do not have to add that extra ounce.
the milage is based on todays habits. up to 30,ooo miles, used to be 60,000 miles.
Some will change less and some will neve rknow they had a rear diff....