I’m going to borrow and make good use of your term: “anti-engineer”. It’s fantastic. Thanks.
First of all, I’m not an engineer. So take what I say with a grain of salt.
You described a simple “homemade” hydraulic steering system on a small boat. If it’s “homemade” it probably was not “designed” by the “anti-engineer” with any particular oil in mind. No need to over think it.
If the oil is red, it’s probably Type A automatic transmission fluid which is common in small systems, especially in cold climates, and especially when the oil is bought by the quart at Autozone. If the oil looks clear or has a greenish tint like new engine oil, it is probably hydraulic oil. The viscosity should be chosen based upon the operating climate. Unless the climate is very cold or very hot, the viscosity won’t matter much.
NAPA and similar places carry “tractor” hydraulic oil in 5 gallon buckets. It is a higher quality, and higher priced, oil designed for torque converters in tractors. In a tractor the torque converter and the auxiliary attachment hydraulics share the same oil. A torque converter really needs this high quality “tractor” hydraulic oil.
I generally use “tractor” hydraulic oil in every hydraulic system. It’s the best regular hydraulic oil that is readily available. The extra cost does not matter in a small system.
In complex, high pressure, high value, hydraulic systems, I use expensive synthetic hydraulic oil that meets the equipment manufacturer specs. I prefer Amsoil products. That would be overkill for your simple “homemade” hydraulic system. The owner won’t want to pay for it either.
When dealing with a small boat and a cheap owner, practical compromises must be made. Save your ammunition for the important items, which hydraulic oil to use isn’t worth using much ammo.