Jimbob, that will work in some but not all situations.
If you have only 6 levels of propulsion, thats what the other player will receive.
If you have 12 levels and he has 5, he will receive levels 6, 7, 8, 9, 10 and 11 !! You are giving him 6 levels that he doesn't have.
Best to know exactly what each other has before making the trade.
I once made this trade with the AI. Can't remember what I gave, but he gave me two levels of fighters. Like this:
fighters
fighters
I only received one level because that was all he had! (Naturally I had to declare war and wipe him out.
)