Gearhead:
I assume the loader has a separate tank and pump that does not also power the three point hitch. The Vickers web site should give you the required viscosity. I suspect that a good universal tractor fluid like John Deere Hy-Trans or Kubota UDT or Caterpillar hydraulic fluid will work fine. For a long time I used Caterpillar fluids in everything because they are clearly high quality, the specs are published, and Cat dealers are usually competitively priced on items like that which can be substituted with aftermarket products.
When that tractor was made (and I can remember then) John Deere and Ferguson hydraulic systems (including those that lubed the transmission as well) specified a straight SAE 20 weight oil, non detergent because there was no filter. Even today, a straight 20 weight hydraulic fluid meets viscosity specs for the Prince PTO gear pump on my FEL mower, and even a 10 weight is adequate for all except the higher temperatures.
The filter should be on the return line just before the fluid returns from the FEL valve to the tank. True, the theoretically best place to filter the fluid is just before it enters the component with the most critical requirements, which is the pump. But the pump needs a free flowing source of fluid. If it has to "suck" the fluid against more that a few pounds of vacuum (3 psi is a typical guidline maximum) it will cavitate and quickly damage the pump. A filter "fine" enough to remove the smaller 5 to 10 particles typically produces pressure losses of 5 psi or more when new and clean, and the losses rise to 15 psi or more rapidly when it becomes contaminated. Cavitation is so harmful to the pump that you would need a huge filter to keep the pressure losses low when it was fresh, and you would even then probably be changing the filter often.
If the tank is clean, and you have a good breather (10 micron absolute or less), the fluid does not get contaminated between the return flter and the pump. When replacing the filter elements, pay the extra money and get "absolute" rated filters. "Nominal" rated filters really have no standard of performance. In contrast, there is an accepted standard for "absolute" ratings. A 10 micron absolute filter removes many times the contamination of a 10 micron "nominal" filter. I assume a filter is "nominal" unless it is clearly stated that it is "absolute". Nominal filters are $5-15 apiece while absolute filters are $35-50 each.
