Chances are they are overfilling it, then the excess oil gets sprayed out of the crankcase breather into the airbox, where it is clogging up the air filter.
Take the air filter out and clean all the oil out of the airbox, then clean and re-oil the air filter, and it'll run fine.
Oh, and as Mike said, don't take it there any more.
If you absolutely have to take it there, tell them no more than 3 litres if they change the oil filter, and no more than 2.7 litres without changing the filter.