A 2510, nice. I have one, but it’s beat up and was repowered. Do you pictures? Where abouts are you. As for the oil, my guess is engine oil like the Cases. I would be interested to know what others say though.
When your Colt was new, the recommended oil was 20W40 but that grade can be difficult to find today. Thankfully, there are other choices, such as, 20W50 or 15W30.... either of which will work perfectly fine in your tractor, as will 15W40. In all likelihood, you are not using the hydraulics to power any attachments so all we are concerned with here is propulsion of the basic tractor.