Most of the 126 cars I've seen use a 14mm allen head socket to remove the drain/fill plugs on the differentials.
Auto Zone sells a 3 piece set(12mm/14mm/17mm) for around $10. I'd get underneath and check just how snug these two 14mm plugs are.
I don't know if yours requires gear oil that supports limited slip operations, but either way, I'd doubt that would cause a leak. I've used Valvoline semi-synthetic in my 126 for the past 5 yrs. with no problem.
My 2 cents.