Mineral oil

On the old D25U 17156 after three weeks of research and two pages of notes  and talking with Ohio Columbus cat that has no books paperwork that goes back that far I finally found Ohio cat in Akron or Canton that deals with the old machines. 
it says in the operation and maintenance instruction manual for this machine to use a MIL-L-2105 requirements or straight mineral transmission oil SAE 90 to the members of this form what have you been using for the transmission oil change

gl 1 gear oil

I went with mineral oil, it is still available today with some time invested. If you are worried about noise or thickness you might consider a gear lube. It's certainly thinner then a gear oil. I considered in my older equipment the increased yellow metals and seals when deciding. The logic of old oils are outdated, insufficient, or lacking to modern oils is totally incorrect directly working with oil engineers. If you want a property in the oil such as tackfulness, thickness, or any other features it is as simple to formulate it upon production. I was shocked to find these older oils no longer have all the downfalls they once had, improvements have been made in production and advancements. Some old oils are considered Green Oils today and meet or exceed other oils on the market today. Let alone some are 95 to 100 percent environmentally safe as advancements in production or better products have been found to produce the oil. These are simply produced with same technology as modern oils, they can meet or exceed the top advanced oils on the market today. Regionally it can be a struggle to find some of these older oils which can make it difficult. Good luck in your search and decision in what meets your needs.

a cat TO-4 spec oil is just fine. i use caltex textrans tdh in our machine and it has TO-4 approval. its a 75w-80 gear lube specifically intended for tractor transmissions, differentials,hydraulics and wet brakes its also approved for and was factory fill on CaseIH-Austoft cane harvesting equipment.

there is no reason you cant use modern semi-synthetic or synthetic oils just be aware how some additives affect the brass and bronze components in these older machines. i dont as i prefer mineral oils and also theyre cheaper too.