1950 MT hydraulics

Anyhow, my question concerns the hydraulics. When I first started, they were strong and had no problem, but after plowing for a while (raising and lowering the plow for each run the length of the garden which is about 60-70 feet) the hydraulics got weaker. If I lifted about 40-50 pounds on the plow, they went up okay. I thought maybe the fluid was low. So, I stopped (half of the garden finished) and added fluid. It was low. I left the plow raised during this time, and it stayed up. About 30 minutes later, I resumed plowing, and the hydraulics worked fine for a while. As I neared the end of my garden (about 6,000 sq ft all together) they got weaker -- I had to help lift the last two times. I didn't have time to recheck the fluid then (will this evening if time allows), but there are no visible leaks anywhere. I will check the fluid again -- could it possibly be low after working what I added through the system? I'm just wondering about the possibilities. I believe I've read about worn pumps not being able to develop the necessary pressure when the fluid warms up and thins out. Are there any other possibilities ya'll can think of?

Thanks for your help.

I have 10W in it -- that's what the manual specifies. I didn't notice the oil being white, but I will check to be sure. Thanks.

I don't know if it's foaming. I'll check this out. I would think that finding such a leak would probably be difficult. How did you locate yours?