Why is my manual transmission so hard to get into gear when I first start the car?

When I first start the car in the morning, I have to use all of my strength to force my manual transmission into 1st or reverse to start driving. However once I've gotten it into gear, it shifts smooth as silk the rest of the day unless I let it sit for a few hours, then it won't go into gear the first time again. Any ideas? 2003 Hyundai Tiburon 6 speed.|||have lighter grade of oil put into gearbox|||== sounds like you need to change the oil in the tranny -----