The differentials are very easy, and any brand of 75W90 weight GL-5 rated lubricant (Without limited slip additive! These are open differentials and do not need limited slip additive, which is a friction modifier and is not beneficial to your Subaru’s open differential operation!) can be used in both front and rear differentials. I would personally suggest using Valvoline 75W90 SynPower for both differentials. A 4 quart jug will do both differentials. Any shop can do this easily. Maybe have your local mechanic do it, or if the dealership doing the CVT is willing to do it for a fair price (under $130 is fair for both differentials parts included) it might be simpler to just have the Subaru dealership do it all at once.
Basic answer is, the differentials are no where near as precise or risky, just a general drain and refill of any GL-5 rated 75W90 differential lubricant (w/o limited slip additive as mentioned above)
Suggestion: have dealership do everything with OEM fluids if you’re happy with total cost, or just have Subaru dealership do CVT and then have your regular mechanic do the differentials, and buy the Valvoline SynPower yourself and provide it for him to use.
Valvoline SynPower SAE 75W-90 Full Synthetic Gear Oil 1 GA
https://a.co/d/4ZnTM2N