While I wish I could agree with Primo's comments, I can't.
All lubricants have improved, but synthetics are so superior in every respect (except cost) that they should be used whenever possible. I can't think of any exceptions.
I do generally agree that deteriorated seals are likely to let lower viscocity, slicker lubricant seep past. That said, I feel, as some writers have offered, that the superior qualities of synthetics may, in some cases, actually cause seals to swell and start maintaining a better seal. This however, is a marginal issue in most cases. Another easy answer - try a crankcase, transmission, or differential full of synthetic for about 5000 miles and see.
Changing it "hot and often" is an excellent battle cry, but one should at least use the best products available.
I will summarize in an even more binary fashion - there is no conventional lubricant that lubricates as well as its appropriate synthetic counterpart. Read the studies, ask the experts.
Let the conventional oil go the way of things like bias ply tires.