Hey James,
You are venturing where few dare to go. I used a 10 weight tractor hydraulic fluid in the ram on my 1941 Hercules dumper. You may need to make your own wrench with some steel plate and a cutting torch, brute ugly but will work for a one time use like yours. McMaster-Carr may have the seals that you need. https://www.mcmaster.com/
Paul


1941 Chevy 1 1/2-ton WW2 4x4 dump truck
In the Stovebolt Gallery
Lots more pictures