So stealing a few minutes from being teacher I put some oil in my type 9. I went to double check I had the right oil and didn't bother as I didn't have much time (and this was gl4 oil so had to be right).

I'm an idiot, I got the diff oil and put a litre in the gearbox. Only realised when I went to open the second litre and noticed it was different to the first.

Is it time to drain the gearbox or will it be ok with 1 litre of mineral EP80w90 GL4 instead of semi synthetic SX75w90 GL4?

As they're so close in guessing it's ok if the mineral oil can take the abuse / doesn't rot anything.

Compounded by putting a hole in the bottle to let air in as it filled so I can't suction it out back into it's original container. Bloody lockdown

It will be absolutely fine. I doubt teh comma oil was about when T9's were first made, they probably got EP80 GL4 at the factory.
The GL4 is the best choice, I am not sure if a yellow-metal safe GL5 would be OK from the syncromesh operation point.

(the one thing I do like about the sx75 90 is it doesn't smell like cat crap when it is a few years old...)

Don't panic! It'll make very little difference. If you notice, you're a better man than me.

Having driven a Sierra that had diff oil in the box I would say change it for the correct oil, gear change was vastly improved. The problem is there isn’t a drain plug , I used some screen washer tubing and siphoned the oil out overnight . One of the vacuum oil drainers would be a good idea

So it was bugging me that I had the right oil. I also have a vacuum pump. Kids outside on a break I started pumping. Didn't get that much out so I took the top cover off and pumped again. Glad I did as what came out was a lot darker (so I hadn't got it all out previously). Now filled with the proper stuff, left a few drops dripping out the filter plug and will reseal on next kids break (betting I forget?). Put the cover on and tightened for now.

Now on the hunt for a new type 9 top gasket & feeling happier it's done properly.

I would also worried about the oil, and would of changed it

You could make the gasket, cornflakes packet or the like, it doesn't really do a lot, and its not under any pressure
and with the cover off, you have the template

I made all of mine on the type 2 boxes, and the ones between the box and bellhousing

steve