You're gearbox is a transaxle.  The transmission and differential share the
same oil.
The differential gears are hypoid gears.  Hypoid gears have sliding contact
and require hypoid gear lube.
Regular engine oil does not meet those requirements.

----- Original Message -----
From: "Brenton Tait" <BTait@INVESTEC.CO.ZA>

This might sound a bit stupid...but what are the chances of using a
mineral 20w50 engine oil in my gearbox? DO I run the risk of damaging
it?