Hydraulic oil for an industial 88

I have an Oliver 1950-51 industial 88. It has a Ware loader on the front and a Sherman model C-8 Power Digger on the back. It has a large Hydreco pump for the hydralics. I bought the old beast to keep around the farm and use it on the little digging projects that tend to creep up on a person. It does an ok job digging but the hydralics are a little lacking. The loader does a descent job lifting but the digger could use a little more power. I had the pump "looked at" and the guy said that the pump was in pretty good shape. I've up dated quite a few of the hoses. It has a transfer block that switches the hydralics from the loader to the backhoe. I notice that the digging slows down as the fluid gets hot which doesn't suprise me too much. I would expect the fluid to thin and lose a little punch as it heats up. It seems like its a low pressure, high volume system and some of the power is lost to the cylinders being worn and so my question is with the hydraulic fluid. I was wondering if these old systems were set up using a little thicker fluid instead of the 10 and 20 weight they sell for hydraulics today. I could see a thicker fluid keeping its power a bit more, but what other fluid can be used in the backhoe. I know some oils can cause the seals to possibly swell. I would appreciate any in put.
Thanks Joe O

Most of the oil use when these systems were new was a SAE 20 non foaming oil.

Kent