G8620-103 (03 Boxster S 6-speed) manual transmission and LSD flush

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Hello, I'm looking for some guidance please.

Apologies for creating a new manual transmission flush thread.
I'm read through hundreds and pages of material regarding this and its all very good information.
I'm just left a little confused when it comes to my Boxster specifically, because I believe I have the LSD limited slip differential because of the PSM in my car. I was specifically not looking to get the model with PSM but when I was recently shopping for my used Boxster this one was the best overall condition, mileage, and least owners, just ticked all the other boxes.

I have some service history for the car, but nothing indicates the transmission oil being flushed or the diff (if there even is one??). My transmission code is G8620-103 and based on other posts, this should be the one with LSD.

So my question is, will I be flushing and filling 2 separate units, one for the gearbox and one for the differential?

Also, my biggest concern after reading for days about this topic, what fluid and what capacity for each:
Gearbox oil type and capacity?
Differential oil type and capacity?

I know the 2 oils to use are Shell Spirax S5 75W-90 and Mobil 1 Mobilube PTX 75W-90

We can't get either here in Canada, and the Porsche dealers (all of them, I called them all) say to use Porsche Classic GL-5 oil, but reading through all the research material this car will require GL-4 only due to corrosion to the brass internals. Has anyone else with the 6-speed and LSD gone this route like what the dealer is recommending?

Sunset Porsche online retailer is one of the rare ones that will ship to Canada and I can only get the Mobil 1 Mobilube PTX 75W-90 from them, but sounds like that is one of the original formulations that went into these cars so I'm fine with that and will be ordering, I just don't know if should only order 3 liters for the manual transmission or if I should order more to have enough to fill the LSD as well?

I would really like to flush and fill the manual transmission on this car since I don't think it has ever been done, I also plan to change the front motor mount and flush the coolant while doing that.

Thank you so much for any feedback and guidance on this.