They are ignorant.

It's gear oil that gets dirty.

I change mine every 30,000 miles (probable overkill), but it's cheap insurance.

Tell them to change it.
They don't want to , then take it to any shop.
As long as they use the specified gear oil, you will be
fine. I mean how hard is it to remove the drain plug,
drain it out and re-fill to the bottom of the drain plug opening?

Sounds like the ignorant arrogant Service Manager at the Dealership that argues with me when I wanted to schedule a transmission service for the
"Lifetime Fluid" 722.6 transmission. He didn't want to do it, then he quoted me $425.00. After some heated discussion I left in disgust.

I now have the special dipstick, and will do it myself.

Steve