MGBV8: This is correct. That is why I have stated 2-3 times that the service life of SynLube is 150K miles, 3K engine hours, or 10 years, WCF.
In my own case, I project that I will get about 105K miles before changing becomes necessary as I average less than 30 miles a day.
You are also correct that for the ultimate result that SynLube should be replaced every 50,000 miles. To put this in proper perspective, this is for someone who anticipates keeping a vehicle for much longer than most people would ever contemplate. For the vast majority of people, the replacement interval described in the first paragraph is the proper choice. I believe Miro would agree with me since other parts of the car would surely be failing so as to make keeping the car unprofitable.