GM steering gear box grease part number 1051052 is what you're looking for. Dealer stock on this item was depleted to zero in 2006 but some restorers and street rodders probably still have some.
Shell Oil makes an identical replacement called Shell Alvania EP 1 Grease MC 2041SIPA. I used some in the Vega steering box in my highboy deuce roadster and it really is exactly like the OEM GM grease.
I hope this helps.
