I am usually in the Shark Tank but since you Mid-year guys have the same box I thought that I would also post the question here. This question was likely discussed before but I checked the archives and came up empty.
What is the best grease to use in the steering box?
GM used to offer a "special" Corvette Steering gear grease - 1052084, but this was discontinued a few years ago and replaced with 12377985.
It's my understanding that the 985 product is just GM's generic chassis grease. The 084 product came in a 14.5 0z. squeeze tube, which was the exact quantity required for one steering gear overhaul, so it was real convenient. The 985 product comes in a grease gun cartridge.
Bottom line is that you can buy the 985 product (and figure out how to install it), but you probably won't do any harm by using a high quality chassis grease from any reputable manufacturer.
For the C1 Vettes, Corvette Central sells a grease for this purpose (with a special Zerk fitting that threads into the box). It uses Pennzoil Multi Purpose EP Grease with Moly #302 (standard 14 Oz. grease gun cartridge). This is a NLGI #2 lithium based grease containing Molydisulfide. Cost is $11.35 (p/n 561165). But this is only listed for the C1 cars.
They list a different lube for the C2 box ("10 Oz. tube of GM lube specially formulated --") . This is $6.55 and is p/n 561170. Don't know what the composition is of this lube.
Duke is correct from what I have come to believe - simple chassis lube. The story as I understand it is that because of the steering gear box's proximity to the exhaust manifold the factory fill would liquify enough to seep out of the box - over time, could leave you with a empty box I suppose. GM's solution was to issue a service bulletin changing the refill substance to chassis lube. Unfortunatly, all of the reprinted service manuals still list the original factory fill product - casting great confusion about to this day.
I still have a couple of tubes of Heavy Duty Multi Purpose Grease Gr.8.800 (Corvette Steering gear grease), 1052084, and it also is in a grease gun tube. When I ordered it 14 years ago, I had to buy the minimun order quantity - 5 or 6 I think. I gave a couple away and I figure I have a lifetime (mine) supply with two tubes. The original had a mil spec on it. It is for applications that require SSG code 5073. Maybe Duke can tell me what that is??
Note that my Helms manual for my 70 Vette specifies 13 Oz. of #1051052 Steering Gear Lube which meets GM spec. GM 4673M. It also states that it should be inspected for seal leakage every 36K miles for "solid grease, not oily film), which implies that it is filed with a grease (not rear end lube oil).
My 66-74 Vette shop manual (produced by Carbooks) states to use EP Chassis Lube.
Bottom line is that any quality EP chassis grease with Molydisulfide would work. The Moly certainly helps in longivity. Note that it is never a good practice to mix greases (specially when they may contain different bases (lithium vs barium soaps, etc., although the majority of chassis greases are lithium based). In other words, remove most (if not all) of the old grease, fill it up with the new grease, and make note of what you used for the next time (5,10, or 15 years from now).