1958 JD440IC hydraulic questions?

First:
My manual only specifies the weight of the oil that goes into the hydraulic system, but not that it is hydraulic oil (as opposed to engine oil). Is it regular oil or hydraulic oil?

Second:
My manual angle blade settles down rather quickly. I don't think I have the spool valve in "float". This happens a lot when I raise the blade and to a lesser extent when I push down on the blade (it easily raises the front of the dozer). I figure the problem is either by-pass in the cylinders or by-pass in the spool valve. If it were a problem in the hydraulic pump, it wouldn't raise the dozer. Here are some things I am going to try to diagnose the source of the problem.

Since the two cylinders use a common oultet from the spool valve, its seems that by-pass in either cylinder could cause the settling (back-flow to the other one). To isolate which one, I intend to remove one cylinder at a time and plug the "Y" at the spool valve. If the blade settles for only one cylinder, I found the problem.

My crawler has a double spool valve and the bottom one is plugged -- not used. I will change the hoses to the other spool valve. If the settling stops, its the top spool valve. I assume it would be OK to run the dozer on this other spool valve except that I give up the "float" position. Are there any other problems in doing this?

If it still settles after going to the other spool valve, could it be some common check/pressure valve shared by the cylinders?

Are there any other diagnostic tests I can run that doesn't call for special equipment?

It has the long tie-rod type of cylinders. Is there a good place to get rebuild seals for these?