hi, I have a clark forklift (approx 6k) It has been modified and painted a few times and and there is no dataplate, but the transmission is a common one I see on other units. I has a high and low gear and a forward and a reverse. Problem is that when I go into forward or reverse, it will ot engage until I increase the rpm's. It also seems to slip when in gear since it has a hard time going uphills. any ideas what to check would be appreciated. Also, what type of fluid should go into the tranmission. I have been told to use hydraulic fluid. thank you