I think I'd mostly be concerned about any residual oil in the lines, since they're probably different and incompatible? But even that should be negligible.

Nylog blue works well for o-rings.

After you weigh it in, close the red valve at the engine, then slowly let any freon in the red hose into the blue hose, and back into the system. Then you won't lose the high pressure freon, and your weights won't be off