I just went through the same thing, I am getting ready to change the oil in my 2000 Boxster S transaxle.
I was all fired up to buy the Porsche fluid until I saw the price compared to other 75W90 gear oils...
I have a couple of gallons of Mobile Delvac 75W90 because I just did the diffs and t-case on my Toyota FJ Cruiser and I had bought extra to do the diffs on my old Tacomas. I wondered if the Delvac would be good for the Boxster, searched and read some threads (some folks have had good results with it), and then just ended up buying 3 liters of the Porsche Transmission Oil.
Out of the places I checked, I found the best price at ECSTuning for right at $30 a liter:
2000 Porsche Boxster Roadster S H6 3.2L Drivetrain Manual Transmission ES#1513799 75W-90 Gear Oil Manual Transmission Fluid - 1 Liter - 99991754600
Suncoast was a little higher at around $39.
I had to buy a few other odds and ends from them too and shipping was only around $9 so it wasn't so bad.
I would have been tempted to try some of the others, but I already have all the issues that I have seen described when using non-oem gear oil (hard shifting when cold, etc) and I didn't want to make it any worse. (I'm not sure what's in there now, I am at 108,000 and I have service records going back 50k miles and I don't see a transaxle oil change so I figure it is time). Too many folks that I respect say to stick with the Porsche oil for this one so I decided to play it safe.
I will report back once I get it changed; I am taking some time off around Christmas so hopefully I will get to it soon.
Steve
Edit: on my son's 95 Miata we used Redline MT-90 because it was almost impossible to find a true GL4 75W90 and we (he) loves it. I thought that was expensive at ~$15 a quart... until I saw the Porsche gear oil... Some places had it at over $50 a liter
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