It will be 5W-30 or 5W40.
5W-30 is VW OEM requirement where as the slightly more viscous 5W-40 is used by most of the forum members as it's ideal for a mapped car.
Generally it's not great to mix oils since they're different viscosity will separate them but it won't destroy the engine.
In the case of mixing coolants (i.e. propylene glycol, ethylene glycol along with their additives to meet G12 and G13 spec). That's a really bad idea, the coolants can mix and coagulate blocking the coolant system. I've seen this on another golf actually with a faulty water pump. The driver kept topping the coolant up with any old coolant they found and eventually it coagulated and blocked!
I would top it up for now and the next time you service it then opt for 5W-30 or 40 as you see fit.