Hydraulic fluid is in the range of 15-20 Wt for pumpability reasons. Additives allow it to have a high pressure retention capability. On tractors using 85 to 140 mineral oil in the differential, they have a separate compartment for the tranny which can use it and one for the hydraulics which use HYDRAULIC FLUID, OR TRANS-HYDRAULIC-DIFFERENTIAL FLUID (THD oil), all being in the viscosity range I stated if you have a newer tractor with a common sump feeding all 3 functions. You mentioned that your hydraulics don't want to work and produce foam....... put the proper oil in the hydraulics and life will get better for you!