This is a *very* good and important question ! .
No worries about the molasses in there now , just jack up the front end so wheels are off the ground , fill it to the top with gear oil then turn the steering wheel full lock , left to right and back again , after a while the oil level will go down as it gets forced to mix with the stiff stuff , top it up again and repeat until it no longer drops , then you're good to go .
This is a Saginaw recirculating ball typ of gear so grease will wreck it , never , EVER put grease in a steering box ! .
You can use STP or Bardahl , any typ of thick oil if it leaks profusely but NO GREASE ! .