Unfortunately, GM stopped making this product about two years ago and the only way to get it is through NOS inventory in dealerships.
It is packed four tubes per case and I got mine from another Stovebolt member who was kind enough to give me a tube. It's heavier than 90W so 90w will be too thin I think. There's a 600w gear oil out there that might work but again I don't know for sure.
I posted the part number here last year and if it didn't get purged from the history, perhaps a search will turn it up for reference.
