Can I just mention that combos suffer with corrosion on the oil sump.
You may need a new one, Kyle can advise you of this, it can be expensive for a new one but there are plenty of second user ones out there that will save you a lot of money.
Also if the garage (or whoever) have cross threaded the sump plug it would of needed rethreading, if not done correctly that would lead to slow oil disposal and will need correcting.
I'm presuming it is an old shape combo.
I can recommend Wirral spares for the sump.