To make a long story short, I`ve got conventional differential fluid in my rear end. Does the fact that there is no breathing tube mandate synthetic fluid ?

 

Don't think it matters. Tube or not, the axle has to breathe or it becomes pressurized when the liquid gets hot. That's not good, things go 'pop'... or worse.

You don't have a tube, but you do have a breather. The tube is so if you sling the car about on a track the axle doesn't leak out fluid, or if you submerge the axle in water, it can't get in as easily. The breather is used because it's easier and most people don't really do much of the above two actions. You can get a tube on it if you want, just a teeny mod. But you can use synthetic or conventional in either case.

 

I was adding fluid to it yesterday and it was pressurized, like it was a sealed unit.

 

Interesting... shouldn't be...

Here's a thread about it I found on google... it's somewhere else, though, sorry:
http://www.allfordmustangs.com/forum...axle-leak.html

And here's the whole 'why' and making a tube and such:
http://www.customtacos.com/tech/index.php?article=86

Ooh, here's a couple on it here:
https://themustangsource.com/showthr...ntial+breather
https://themustangsource.com/showthr...ntial+breather

Why it'd be pressurised... Faulty breather maybe. You should have that checked or replace with the tube anyway.

 

When I pulled the plug I could hear it hiss, I`ll check it out.