Trying to sort a problem with my dad's 02 plate tdi passat (5spd)...

Gear change has been getting progressively worse for a while - decided to check fluid level - removed undertray and noticed that the underside of the box was wet with oil. (Could be engine oil) Removed 17mm allen plug from beneath speed sender on nearside - cant touch oil inside am I right in thinking the fluid should be up to the level of the bottom of this plug hole?

Is this a common problem? Is there likely to be a particular place it's leaking out? Is the transmission a sealed unit or is there an external cooler?

If I can't get trans oil from a dealer tomorrow (Friday) does anybody know what specification oil it is?

Thanks.