Yesterday I drained the gearbox oil. It had the same pungent smell of GL-5. The GL-4 I bought did not have this same smell. So my guess is the smell is sulfur.

So I'm worried this stuff may have shortened the life of my syncros. Like is has so many Fiat gearboxes.

Also the oil was very thick, black and sludgey.

I considered filling it with diesel and running it up the road and back to clean it up. Or fill it fill of diesel and letting it sit overnight and draining it out.

I'll probably put a few miles on this oil, drain it and replace it.

By the way, this oil says Hypod but does say EP. Is that the correct oil?

https://www.amazon.com/gp/product/B000M ... PDKIKX0DER

Both GL4 and GL5 gear oils are likely to have sulphur or phosphorous-based extreme pressure (EP) additives, the difference being that there will be more of them in a GL5; typically 4% in the former and 6.5% in the latter.

At all costs, do not run one of these gearboxes with diesel in it, or any other solvent such as paraffin, petrol and so on.
Their resistance to extreme pressures is perhaps one-third of one-half of s.f.a...

The crownwheel and pinion gears in an A series gearbox are of spiral bevel form, so an EP oil meets its requirements for lubrication.

Fwiw, I think there's an awful lot of fuss made about the dangers of GL5 oil in an A series gearbox.
I've been using the same gearbox in my old Dyane for 20 years now, it's always worked hard, usually towing a trailer with up to 5 chassis and at one point was covering up to 25,000 miles in a year.
For about 5 years, I used GL5 grade oil in it and the synchros are still fine...

Keeping busy with chassis in January by slcchassis, on Flickr

"Thick, black and sludgey" oil is the real threat to a gearbox's life, not GL-5 in stead of GL-4.

That being said, when buying new oil, why not do it as right as one can and go GL-4.
I think the oil you found is ok. A bit on the thick side, but I have read it's ok on old boxes.
Myself I use a cheap but well known brand, GL-4 80w-90 or 75W-90, mineral oil.

And I change it before every 2CV World Meetings