Owners manual says once every two years or 24,000 miles. Local Porsche dealership and Porsche NA customer service says 1 year or 12,000 miles. What do you do?

 

lol. 24k miles.
do it once a yr at a min regardless of miles

 

According to Porsche all 958s have a 10k/1yr OCI. I know that's wrong though as the diesel is 5k.

There is an option in the MFD that will tell you how much time and distance you have for each interval. That's what you should go by (though depending on the age of your vehicle there could be a campaign that changes the intervals (the original diesels came with a 10k OCI, but it was updated to 5k later)).