You kind of lost me on the "Does the clutch run off same tank as hydraulic fluid for attachments ??" That is a dry clutch, and the hydraulic reservoir sets under the gas tank. There is no oil reservoir in the torque tube.
There is a small square head, transmission level check plug behind the right axle housing. I believe it is just under the center line of the PTO shaft.
Most auto parts places do carry 80-90 wt oil. If you want to drain it, and refill, it will take 5 gallons. If you drain it, and the oil looks milky, it would be best to get several gallons of cheap automatic trans fluid, like Dexron II. Pour that, plus a quart of isopropyl/ rubbing alcohol in the tranny, then drive it around for a bit. The Dexron II is high detergent, and will wash the interior of the trans down. The alcohol will remove any water that may be in there. You can even throw in a gallon of diesel fuel if it makes you feel better. After driving it around for a bit, drain immediately while crud is suspended in the mixture. Let it drain completely, then add the new trans oil.
For the hydraulics, Hy-Tran is the OEM brand oil, but a good auto/truck supply should carry a Trans-draulic fluid. Just make sure to read the label, and make sure it meets Hy-Tran specs. It is pretty thin, comparable to a 10 weight oil. A lot of guys use 30 weight non-detergent in the hydraulics, in case the seal in the hydraulic pump may be leaking, so as not to dilute the engine oil with Hy-Tran, or it's equivalent. But it could make the hydraulics pretty slow in cold weather. I use a Hy-Tran equivalent myself, but I do check the hydraulic level frequently, and engine oil every time I use it.