Hygard gets darker with age if you use it or not. I’ve had people give me old unopened bottles of hygard with tractors I bought from them and when opened it looked more like 15w40 then hygard. I didn’t use it. It must be one of the additives either oxidizes or otherwise changes with time.
I have seen the same thing. The fluid darkens, but doesn't smell burnt or "roasted", Also, its viscosity seems the same and other than the color, it doesn't seem "different". But I also chose to pass on the use of it.
The other thing I have seen a couple of times on machines which were never serviced is after 20 years, the inside of the filters sort of "fell apart" and plugged the filter by all of the filter panels sticking together. I have a filter inspection tool for opening the filters as we always inspected our racing engine oil filters. That's where the real value of a filter is seen and why paying for a quality filter is always a good investment and cheap filters are often cheap for a reason...............
I also agree with the other conclusions about the possible linkage issues, which is why I attached the illustration from the parts book. I would suggest carefully inspecting the linkage and seeing if something is broken, missing or gone. Also check the drive pulley on the unit. If mechanically all of the pieces seem functional then I would service the unit and give it a try.
There have been a couple of machines over the years which I have encountered similar to this situation and a filter change and fluid flush were able to get the machines going again. lt's worth the shot.
I do hope the OP posts picutres of the fluid when he drains it out so we can see it. I also hope they stay in touch and let us know what they find so we can help further. I have seen other machines go this long without filter and fluid changes. It's not what we like to see, but what's done is done and lets do what we can to remedy the situation.