moeart, I see you are getting some good advice here but I see a few things to add that may make things a lil bit easier to understand.
The difference between gear oil (85-90wt) and hydraulic fluid is the weight of the oil. Gear oil is thicher and heavier weight, used in the older tractors in the transmissions, rear ends, gear boxes on equipment such as the gear box on your bush hog. Hydraulic fluid is more like transmission fluid in a car, it is thinner, lighter weight, not as thick and heavy. The newer diesel tractors usually use what they call JD303, which is basically a hydraulic fluid. The older ones usually use the 90wt gear oil.
I am going to guess that someone told you to run the tractor in a certain gear and set you up to do some bush hogging and it worked well for the situation you were in at the time. But now you are in thicker grass or tougher conditions and you are still in the same gear as before but it is not working. Whoever taught you did not teach you how to judge your area and change your set-up for the job at hand. Sometimes I bush hog in high range, sometimes low range, all according to my conditions. You want your bush hog to turn at 540 RPM's, running freely and not bogging down. I refer to it as "singing", keep the blades turning and the bush hog singing at all times. You can judge the transmission speed and range however you feel is appropiate as long as you keep the bush hog singing, cutting properly. Some of my areas are thicker than other and sometimes I let it get higher before I cut it, all this goes into mind when setting up to cut. If it is thick and tall, I run it in low range and maybe second gear, if short and thin, I may go to high range and first gear. I have run it in high range, second gear and even slow down the PTO rpm's if it is thin enough grass. The thing to keep in mind is if it is cutting properly and not bogging, not having to go over it twice, then whatever gear you are in must be the right one. If on flat smooth ground and the grass is thin, I can move along faster. If it is rough ground and thick, get it in a lower gear and move slower and just "grind away" as I call it, still giving me a good finish cut. Cutting the same areas may differ, all according to how high you let the grass grow before you cut it.
Keep in mind, keep the bush hog singing, select the fastest speed you are comfortable with AS LONG AS you are NOT bogging the tractor and getting a good clean cut.
Now go play with your tractor, learn what we are talking about, get the feel of it, and come back and tell us you got it all figured out.
Tractors are dangerous, be careful. I know of a guy that got off of his tractor, left the bushhog running and stepped on a heavy vine. The bushhog grabbed the vine and pulled it and him into the bushhog destroying one of his legs. I never thought about this until it happened to him, so be careful, but get out there and get the feel of it.
The term "new at tractors" suits you best. Never be afraid to ask and learn what you can. Hands on experience is the best. Just because you are a female doesn't mean you can't learn and be good at it. Give yourself credit where credit is due. I like a female that is not scared to get out there and get her hands dirty as long as she remembers she can clean up and still be a lady when the time calls for it. High five to ya for getting the job done.
