Though this is not really conclusive or anything, just an observation I have made from my engine, when I have used 10/40 or 15/40 semi synth, the car has seemed absolutely fine.
A few months back, I changed to some 20/50 stuff and after quite a few miles under some high revs, the oil light began to come on.
At first I thought it was a more serious problem, but after draining the oil and putting 10/40 back in, the light no longer comes on.
So am assuming that maybe the 20/50 mineral oil could have become less viscose under such repeated heat?
Remember, this is merely a one off observation I noticed, and could easily be just a matter of coincidense or stupidity...