Don't overthink it. The commonly used Lillywhite sewing machine oil is basically hydraulic fluid (oil), from memory around ISO 66. I couldn't get ISO 66 locally when I was looking so I bought a slightly heavier ISO rating and it works fine. Or you could make up your own concoction like kgg did. Auto trans fluid is very versatile, btw, I've used it to lubricate clock mechanisms and clean/lubricate my rifle barrels.