Automatic gearbox fluid

I have a 1997 Mercedes-benz C180. The fluid for the automatic gearbox is Dextron lll, but my garage used Comma Automatic Transmission + PSF MVATF Plus semi-synthetic. Is it possible to use fully-synthetic Comma Automatic Transmission & PSF MVATF plus? The car takes 6.6 litres, but you can only drain four litres, leaving 2.4 litres in the torque convertor and pipes; will the fullysynthetic fluid mix with the semi-synthetic fluid left in the system? A Wray

The official recommendation for your C180 is A 001 989 77 03, although Mercedes does also list other compatible oils, such as RAVENOL ATF M 9-FE. The Comma oils do not appear in the Mercedes listings, but according to my data, the Comma Automatic Transmission & PSF MVATF Plus is the correct oil for your transmission. As you say, when you drain and refill the box there will be a percentage of old oil left in the system.

While the transmission requires a specific lubricant to satisfy the frictional characteristics of the gearbox, as long as the oil is the correct specification then the semi-synthetic and synthetic versions of the oil can be mixed together. One good way to reduce the amount of oil left behind is to drain and fill the gearbox, run the vehicle, then repeat the process. This will ensure the majority of oil has been replaced.