1959 John Deere 430W

I have a 1959 John Deere 430W that I bought @ 5 years ago from a friend. He used it for parades, I put a 5 ft John Deere bush hog (MX5) on it and use it to mow pasture etc. The previous owner had converted the hydraulic oil from the heavy oil they originally used, to the JD Hy Gard per dealership recommendation. I have used this machine each summer with that oil and it has worked but there is a slight leak from the shaft where the lift lever enters the transmission case/differential. I just put a catch basin under it and add oil as needed. Per the manual, I add it till it starts to come out of the petcock located on the right side of the differential. It has always had a slight hesitation when lifting the mower but last year it got really slow at lifting the mower. This year it is obviously no better as I haven't done anything to fix it. My questions are: Is there a screen in the hydraulic system that could be cleaned, if so I don't know where it is. Then, should I switch it back to the original recommended oil instead of the Hy Gard? Should I add more oil after it starts to run out of the petcock and if so how much? I said the leak is at the hydraulic lever, but I guess it could be from the rock shaft on that side too. I have read that replacing the seals on the rock shaft don't always help because the shaft may have become egg shaped. I'd like some advise on how to get this machine back to operating properly. Thanks for any help in advance.

Hyguard is fine if you rebuild the TOM lift and pump. If you don't it leaks more and isn't as responsive.

The correct way to fix it is to replace all the soft parts (seals and gaskets). And it will probably still leak. Personally I would drain out all the hyguard I could and replace it with 30wt non detergent oil. It works better and leaks less.