Changing the fluids this weekend, new some tips on what to choose (Diff, Trans, &Oil)

[08Chevy]

I use to go with Mobil 1 synthetic for my oil but now I've heard they switch to a different base oil which changes it up or some sht.

1) What brand oil do you use and what weight? I know I will probably go with 5/30 like usual and won't ever change. Probably going to buy Royal Purple if Autozone has any.

2) How many quarts do I need for the trans and what brand/weight do you use?

3) How many quarts do I need for the diff and what brand/weight do you use? Do I need the G80 additive too or is there a double mixture with the diff fluid that already has it?