Transmission help

Hello guys, I was hoping you could give me some ideas on something. I have been trying to help my brother in-law with his first car and everything is pretty fixable on it but the main problem is the transmission.
When he first bought it he mentioned it could only be driven in first gear, and it did but not very well. Of course the first thing I did was have him put it in each gear for about three seconds each and throw it back in park to check the fluid (which I thought was the best way to check it) and the fluid was just below the hex marks but I did not think it was low enough to cause that big of a problem. So we decided to try some Lucas transmission fix and it did a lot better but not as good as we had hoped. The first and reverse gear worked perfectly, the second gear did better but would only push it slightly when throttled, third gear did nothing and drive gear did nothing. And when I put it into drive it does not seem to even engage, like the rpm's did not lower, it did not click or anything.
I know when it comes to a bad transmission there is not much that can be done, but I did not know if there were some things I could check out that could help it? Any suggestions? Thanks!