I've llked thru the owners manual for my 2016 CX5 and it doesn't mention changing the transfer fluid unless the case has been immersed in water. Does that mean the fluid doesn't need changing or is there a recommended interval?

 

I doubt any fluid is "life time", so if you are changing the front and rear differential, you could change the transfer case oil as well.
For peace of mind if nothing else. At the very least, every 'other' differential change.

My 2¢

 

I changed mine at 80,000kms.

 

The recommended fluid change for "average vehicles of all makes" is, 50,000 kilometers or 30,000 miles.
Frequent towing, especially heavier weights, slap happy paddle shifting or stream swimming can reduce those mileage figures.

 

All Wheel Drives have clutches inside; part of the 'Auto' engagement system. So changing the fluid is an important
maintenance item.

My 2¢