645B Transmission problem

Hi everyone - I'm new to the forum and have a question. I've got a Fiat Allis 645B wheel loader that we picked up last fall. First decent sized machine we've ever had (had a Michigan 75A previously) and the machine lifts very well and stops very well but really doesn't seem to push that well, or at least as well as I think it should. We were doing some land clearing with it this fall on flat ground and alot of the time the machine would hardly push its own weight just with the bucket riding on top the ground. Very difficult to dig much if any dirt with it. Where do I begin to troubleshoot. Also it seems to get rid of a decent amount of oil too. What should a guy use for oil? Is there a way to test the charge pressure? Or what else should I try. Gauges shows in the green according to the gauge on the dash.

Thanks for any help, also does anyone have a good service/repair manual for that loader?

Thanks.
Chris