W.salam!
We have a 5kVa generator and it uses 1 Litre of oil. You should add the same quantity in your generator's engine that you added previously.
You should drain the remaining oil from engine as well and then pour new oil in it.
Be careful in the future as if the engine gets weaker your generator will not be able to bear enough load.
I can only give advice regarding engine, rest you should ask the generator supplier.