Grease is a lubricant, but my opinion on its own, it is a better initial assembling lubricant rather than a long term lubricant in a steering box as I feel that it isn't "slushy" enough to make its way into all areas of the box simply by pumping it in.
In the workshop we used to make a "slurry" up of 50% gearbox oil and 50% Molybdenum Disulphide grease ( try saying that after a few beers

) which is the same grease commonly used for CV joint packing.Grey, filthy grease, and hard to get out of the pores of your skin but works well.
Then we would pump it into the steering box with a huge syringe from the local vet that is used for injecting medicine into bulls bums.
You could also buy a special grade gearbox/diff oil in 250 grade viscosity ( or higher ) which is a thickness like warm honey.It will make its way into the needed areas especially after a long run when the steering box is warmish from underbonnet heat.
I replaced the seal in my steering box about 18 months ago and just used straight 90 grade gear oil.No leaks so far from a new seal and using 90 gear oil.
I would have made up the Moly/Gearbox oil slurry or used 250 gear oil if I'd had the time to do it when I fitted the steering box back then.
I have the part number here somewhere for the aftermarket steering box seal that fits perfectly and from memory was about $7 from the local bearing place.
Let me know if you want the part number and I'll see if I can find it.
I can't think what to write here so this will do.