My AC clutch gave up last week. Had been making some noise for a month or so when first started and the coil just burned up. Since the compressor was a 6 year old rebuild and a new clutch cost close to the compressor and clutch unit I replaced it all. The car had been converted to 134 by previous owner and I had replaced the 134 with duracool two years ago with pretty good results, way better cooling than with 134. Since I had to open the system this time I decided to flush everything, replace the drier and refill with mineral oil and r12. What a DIFFERENCE! It has never been this cold at an idle. Even when the car has sat in the sun and is 120+ inside the air from the vents is cold. It cools down in about half the time as before, and on the highway I noticed the compressor is cycling much more than before which made a difference in fuel consumption of around 3mpg.
If you need to replace the compressor in your 300d then spend the extra money ~ $80.00 for r12, it is worth it and what the car was designed for.