I'm sure the viscosity is pretty similar. I could see that if too much grease was packed into the guide for the rack that you could create some suction that you'd have to overcome.
If I remember correctly, once I got the rack reassembled, I could move the rack side to side by turning the input that the steering shaft mounts to, by hand. It's still very hard to turn by hand, I don't think it should be impossible.
Another test that's pretty easy is to reinstall the rack on the crossmember and slide the steering shaft coupler back on. You can then use the steering wheel to ensure you have a smooth operation. Without the tie rods connected, the wheel should feel feather light.