Depends on what you call dirty. If it is full of dirty black oil, change it and the filter for the recommended oil and keep a watch on it. Generally dirty engines are a result of poor driving and incorrect use of the vehicle. For example, a vehicle that is continually operated cold and for only short distances, will inevitably build up sludge, especially if the oil changes have been missed. This is because water is a byproduct of combustion and the engine never gets hot enough to evaporate the condensate that builds up. This condensate reacts with oil to thicken it and also can provide a false reading on the dip stick indicating full or over full condition.
If it were mine, I would change both oil & filter myself, add the correct amount of the best oil you can buy and then you know it is fresh and new. Then I would arrange for a nice drive @ at least 100 kmh for a couple of hours to power the oil through the motor.
Do not do country driving on the dirty oil as the heat form the engine will evaporate the water and you may be left with a very low oil level in the sump.