SOS Massey Ferguson HELP--Update

My dad's tractor-repairman friend came over, did his thing, and left again. The tractor still isn't working, but that's because of a separate issue.

To review the original issue, long story short, we were trying to change the oil and oil filter in a 1965 or older Massey Ferguson 135, with a continental gasoline engine. Everything seemingly went fine, until we got to actually putting the new oil into the tractor.
My 83 year old father instructed us to put the oil into the hydraulics system instead of the oil tank, and chaos afterward ensued.

Now: tractor-repair friend comes over, and assesses the damage... Which is apparently nowhere near as bad as I was fearing? I'm honestly feeling a bit conflicted about this, ya'll. Tell me if you think this makes sense.

He says that in a Massey Ferguson tractor of this age, it is TOTALLY FINE to use motor oil in the hydraulics system IN PLACE of hydraulic fluid. Like, interchangeably...

If that's true, my dad is a lucky dude, (and has now officially learned no lesson from this whole affair), but I have questions that I didn't think to ask at the time.

1. Even if hydraulic fluid and motor oil are interchangeable, does that mean its ok to mix them?

Because there was already hydraulic fluid in there, and now it has a gallon of motor oil mixed in with it. And I have no idea how much hydraulic fluid was in there to begin with. So... Just a completely unknown ratio.

2. Should I just drain the hydraulics completely and put in straight hydraulic fluid?

IF the oil and fluid CAN'T mix, then I'd probably have to flush the tank to make sure the motor oil completely drains, right? (I hope not, that stuff is soooo expensive!!)

Anyhow, moving on: after the whole hydraulics vs oil revelation occurred, repair-friend goes on to ACTUALLY put the oil in the oil tank. (It should be noted that at one point in the past, I asked my dad what that part was for, and was told "that ain't nothin. It don't do nothin." so HAH)

We fill the oil. We do the whole start the engine, then turn if off thing to get the new oil into the system. All's good. We start the engine for real. The oil filter starts spewing out oil from the base. Back on the day in question, when we first changed the oil filter, Dad had insisted we use the same gasket/o-ring thing that had been in there for God knows how long. Obviously, that didn't work. We now tried using the one that actually came with the new filter, but its too big to fit into the little groove on the physical tractor body. So now we're just trying to find the right size o-ring. Somewhere. Napa Auto Parts, maybe?

In the mean time, what do ya'll think? To drain or not to drain, that is the question. Does tractor-repair-friend know what he's talking about? Please let me know!

Don't worry about the gallon of motor oil in the hydraulic system, it will be fine. One of these days, after you have run it some and find out if it needs other repairs, you will likely want to give the entire tractor a complete good service, change the hydraulic oil then. Until you do, don't even think about it.

I will echo what Jim said and I will suppose the majority
of posters here will agree that no harm will come of it.
Outside of the motor oil being a bit thicker and having
different additives that address the type of wear that is
seen in engine instead of that for gear train wear there
is no real deference. The oils will mix fine and will no
separate. Here is a link for others to your previous
post.
Previous post

Absolutely nothing to worry about. If he added antifreeze
to the engine oil, Huston we would have a problem.

Special hydraulic oil is not needed in any tractor prior to the use of clutch packs

Do you have a Massey Ferguson dealer close by? If not, Steiner tractor might be of some help. (in finding an O ring)

I just checked with Steiner. I think they have what you need. They say they provide an additional gasket with their filter that oil filter mfg do not and it is needed to
properly seal the filter. That's all I know.