Hmm, if the oil t-stat has indeed opened and it's clear even with the oil cooler in the loop, that's impressive. But the oil t-stat opens at a higher temperature than you'd think - not related to coolant temp. Only way to make sure is to feel the oil cooler & lines, they will be HOT when the t-stat has opened after a hard drive. I experimented with this once and was surprised at the results.
Of course MB designed the system so that this flushing isn't necessary, the leftover old oil is OK, but the concept sounds neat. Would cost a small fortune to flush with synthetic, which is about all that's in my system for the last 15-50kmi (depending on which car).
There's little (if any) functional benefit to all of this, but as mentioned, it could be a plus while selling a car!
best regards,