Hi - I have a 1977 IHI Shibaura sd4000a 2 wheel drive. The hydraulic arm is going up but won't stay up when I disengage the drive. I think the hydraulics and gears have the same oil top up. When checking the level it is good but creamy white. I want to drain it and refill but am not sure which oil/fluid to use. I have read that for the older tractors some of the hydraulic fluid used these days is too viscous. Any ideas on which fluid? Thanks.