Reply to dick:
From form #BG-500 dated 5/53. Use good grade of motor oil or hydraulic oil - In Summer S.A.E. 30 Visc. @ 100 F. -500 S.S.U., Visc Index - 80 Min. In Winter S.A.E. 20 Visc. @ 100 F. - 300 S.S.U., Visc Index - 80 Min. Below Zero ( Same as winter but Watch Pour Point, and warm up equipment slowly. Note: Use an oil with a HIGH VISCOSITY INDEX, at a sacrifice of other characteristics. DO NOT use hydraulic brake fluid, aircraft hydraulic fluid, low viscosity naphtha base motor oils, or oils that have a tendancy to foam. We used in later years ATF to work with the rest of our equipment and found it satisfactory. I have several different arrangements of Be-Ge's and used parts available.
All of above will work if it was closed system and only did the dozer. But when you hook up the remote hyd. to anything that is shared with modern tractor you are useing that requires torque fluid the next time you hook up to newer tractor after useing this one you will contaminate the its hyd system and eventully cause internal problems in newer machine. Torque fluid or called (tractor trans. fluid) is what is used in most newer machines as the have only 1 system that runs the whole tractor(trans., internal hyds., and remote hyds.) and it takes much higher temps without breakin down which is required in the trans and close tolerances of the newer machines.
Any wear in your old system most likley came from getting hot and cheap oil breaking down and not lubricating properly not from just using other oils alone. Oil only causes wear when it gets hot and breaks down or gets contaminated.
The heavy stuff you had in it was old trick,put in heavier oil to make more presure out of worn system.
So spend the extra buck and buy the good stuff.