I still have the original shocks on my Durant 60. What kind of oil should I use in them?

The only thing that I could find was Hydraulic Oil from a farm machinery outlet. Try TSC if you have one near you.
The original oil was a mineral based oil.

For your consideration & information use:
I have a quart can in front of me that reads:
Kushion King - Shock Asorber Fluid
For Houdaille Shock Asorbers
from Western Auto Supply Co.
It looks to be about 40 weight oil.
Also consider a motorcycle supply company. Harley- Davidson uses shock fluid in it's telescoping front forks.

After rebuilding the original shocks on the front of my 4-07, I've had success using automatic transmission fluid. It's a high detergent oil with good anti-foam characteristics.

RonJ ...

Where Are You From? www.ronjpics.com/durantstuff.htm

I also used automatic transmission oil in my shocks for my 1930 614 Durant after I rebuilt them. Seems to work really well.