gear slippin

hey guys I have a problem with my car this car had been sitting for about 2 years give or take and I took it out for a drive the other day and i noticed that either 2nd or 3rd gear slips. can this be due since the car hasnt been driven in that long? if so what can i do to fix this? today i added some lucas oil that helps with leaks,slips and grinds. should that be enough?

ps - i took it out for a drive today about 1-2hours after i added the lucas oil and it was still slipping maybe it has to sit longer?
 
Are you sure that the fluid level is correct? To low or to full can cause slipping issues also. Be sure to check in park fluid at operating temperature.:D

I believe it was at level like where it says Full. but ill have to check it tomorrow since the car its a my dads buddys lot.
we took it for a spin today and I flushed the radiator and i guess i didnt tighten the water hose completely that when we got there it was dropping the anti freeze and we had to put water on it until it would stop throwing the water back (steam) hopefully nothing broke :D