In Reply to: Re: Hydraulic Fluid Source posted by Tim Daley(MI) on February 25, 2019 at 07:31:52:
You have your part numbers confused - llamas had them right but missed the mark on their properties.
Ford tractor oils from the 50's:
M-4864-A - Mild EP SAE 80 gear oil
M-4864-B - Mild EP SAE 90 gear oil
M-4864-D - Ford loader oil - properties not specified
Ford Tractor and Loader Oils
The M-4864-D oil was a loader oil and as was common in that era likely contained sulferized Sperm whale oil as an EP component. As the Ford chart shows it could be used to thin M-4864-A (SAE 80) in the hydraulics of the NAA and Hundred series in cold weather. So we can safely deduce it's viscosity was lower than SAE 80 gear oil. Exactly how much less is a harder question. Viscosity grades for gear oils are defined at 100°C and modern hydraulic oils are defined at 40°C so a direct comparison is difficult. But assuming a typical viscosity index of 95 for a 50's era SAE 80 gear oil (M-4864-A) we can compute an operational viscosity of roughly 70 cSt @ 40°C. That corresponds to an ISO VG 68 hydraulic oil. Hence my suggestion of an ISO VG 46 oil as an equivalent for the thinner M-4864-D.
If you can find any documentation for any of those Ford oil specifications you should publish them. I prefer hard data to speculation and I have searched long and hard. That chart is the most definitive information I have found.