Giant VT3 - manitou black comp fork problems

Hi guys,

I have recently purchased a giant VT3 mountain bike with manitou black comp 90-120mm forks attached.
The bike has just been serviced and is in very good condition (05 i think) but find the forks bob too much. Im using the bike to regain my fitness and will be used primarly on flat gravel roads.
The forks dont have locks out on it and my question is can i modify these forks to become stiffer or do i need to purchase new forks? If so what is a good fork for my use??
Can i had any hydraulic oil or do i need specific fork oil. If i over fill the forks will this help make the forks stiffer without damaging the fork??
Any help on this matter would be very much appreciated. As you can probably tell, this is my first xc/full suspension bike but will not be my last!

Cheers,