SAAB 99L 1974 Manual Transmission Oil?

I have a SAAB 99L 1974 that has a knocking sound coming from the transmission when starting. You can only hear it when the car is in gear and neutral, however when you push the clutch in the sound disappears. The sound does eventually go away, once the car gets to normal operating temp it disappears. And the colder it gets outside the longer the noise is made.

So, I want to start with the easy stuff. I want to replace the transmission oil, and hopefully this will solve the issue, maybe just something is dry in the warming up phase and this can fix it. I pulled the nut from the transmission and noticed the oil is, some what low. Well most likely very low, the previous owner never replaced it and neither has the owner before, so god knows how long this thing has been running on this oil. It looks somewhat clean, but it was a few mm below the cap and I was on a slight downhill, so in reality the oil should be dripping out.

Anyway, to my question, I can’t just put more oil in it, I’ve got no idea what’s in it, smells and feels like transmission oil, but I’m not mixing. So I started looking what oil I should use and I see people saying using just normal 10W-30 engine oil is better to use and was recommended in a SAAB bulletin back in the 80’s. So to my question, what should be the new oil I put in the transmission?