The writer is asking a valid question.  

He's about to take the car for a steady-speed one-day trip of 1,000km.  If the oil is fine, why NOT change it after the one-way trip?

I've heard of people servicing their car immediately before a holiday trip and running into problems with the work done.  The reader is asking a valid question, and if there is sufficient life left in the oil (which it surely is as Mercedes mandates factory synthetic oil at $20/litre...), then there is easily enough life for the oil to travel well in excess of 1,000km of easy highway driving, before the next oil change.

as per my other comment, one option undertaken by enthusiasts is to change the filter only, and top up the oil.  As it has been shown that the Filter will reach its capacity far earlier than when a synthetic oil will exhaust its additive package that serves to protect the motor.