your oil change was 4000 kms ago and you are getting a service A warning?? I think the last mechanics never reset the service counter. The service A counter shows up at about 15000 kms. Keep on using mobil1 0W40 - its a very good oil for your car.
The A/C in your car is basically simple like every other car except for the compressor, it uses a PWM solenoid to vary the compressor volume/stroke depending on the system load. Usually the reason for failure in these cars is servicing by ill equipped personnel, e.g. using a thicker oil in the system or lower quantity of the recommended oil will kill it shortly, similarly if a shop uses their "fridge ka compressor" to pressure test the system - they basically push moisture/oil vapour and other contaminants into the system - essentially destroying it. A tank of nitrogen is used to pressure test, its clean, inert and dry. Make sure your A/C mechanic has this equipment and knows how you use it, and make sure they use a dedicated vacuum pump to pull a good vacuum before charging the system.
It also requires correct charge by weight as the compressor is variable type, the gauges will behave very odd and will confuse the mechanic if he doesnt know this.
For diff oil - its your choice - you can change it out now - and this current fill will be ok till 200,000 kms - MB diffys are quite large capacity and dont break down the oil much except the first fill.
A service I always recommend is to flush out the brake fluid - its the most neglected fluid in your car and the most safety critical too, A litre of DOT 3 or DOT4 is all you need and a sensible approach to change it out.
