This one's been bugging me for a while now, so I've finally decided to ask it here. For whatever reasons, everyone seems to be OK with mixing Mobil 1's 10W-30 & 15W-50 oils together, but not other grades. Why is that? What is it about these two very different oils that the same people who say "don't mix" have no reservations about mixing these? I guess they're kinda close at the xW- end, but they're worlds apart on the hot end of the scale. And their vis spread is not even close to one another, right? 20 pts for the 10W-30 versus 35 pts for the 15W-50. Doesn't it make at least as much sense to mix oils with similar vis spreads? Like M1 0W-30 & 15W-50? Or M1 0W-40 & 15W-50? Or D1 5W-40 & 15W-50? Or M1 10W-30 & 0W-20? Also, reading thru Mobil's literature, it seems as though the features & benefits of 15W-50 more closely match those of the 0W-40 than those of the 10W-30. I'm not promoting "home brewing" our own blends. I'm just trying to understand why one pair is accepted while others are rejected.