do I really need to swap out any rear axle oil?
I was searching for my vehicle and found this page:
https://www.searchforparts.com/oil-c...-tucson-2.4l-l
and offers 75-90W oil for my Tucson, 2019.

any thoughts on this or do I leave it as is?

 

Yeah for sure!! I change most the fluids,
I got a new Tucson 3090km and I've already changed the engine oil twice, moving forward I'll change it every 10,000km.
Rear diff and transfer case I'll service every 50,000.
Same for transmission every 50,000 drain and fill only, filter I'll change every 1000,000
Coolant I drain and fill often around the same time as the tranni
I always drain and fill no flush,
Love the royal purple I use it too, it'll work great in your diff
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Yes you should change ALL the fluids including the differential and even the transfer case if it's an AWD.
But only if you plan on keeping the car for a long time,
If you're just keeping it for a couple more years then I'd leave the diff fluid alone.
​​Amsoil in the link you provided is an excellent choice.
I have a brand new Tucson and will be first changing my diff fluid at about 15,000.
And after that every 3rd or 4th oil change I'll do the diff, transfer case, transmission and coolant.
No not a flush, for any of them,
Just drain and fill.
Tip: always measure what you've drained out, especially with the tranni fluid.