Used to be "Check every 50,000 miles" but that was before the amount of exploding diffs became a problem for Alfa.
Now it's check every 24,000 miles and replace every 48,000. Still far too long IMO... Diffs are still exploding.
I change the gearbox oil in V6 and 6 speed JTD's every 2yrs or 24,000 miles and check the levels every 6,000 on all of them I look after and work on... Never had a diff problem ever for some strange reason... Can't possibly think why???
Know what I mean?
It's only 2 litres of 75w 90 GL5 empty to full with the drain plug at the bottom of the case and the dipstick on the six speeds is located on top of the diff... Difficult to reach but possible to check in 5 minutes (cold, engine off) Refill through the dip stick housing tube.
Something tells me your grumbling may disappear assuming that the damage isn't too bad. It won't (or shouldn't) help with a slipping 6th but that's a separate clutch wear issue
For those who have the Q-system and may read this thread in the future during a search....
Its 3.8 litres of Dexron II or III empty to full on the auto box. Change every 24,000 or 24 month regardless. Check level every 2000 miles and should be checked warm with the engine running (dry sump method) and refill via the dipstick tube at the front of the battery area