Do a google search for 'mobil 1 10w-40 15w-50 pds' . There's one product data sheet with both oils on the same page for comparison. Look at the high and lower temperature viscosities and you'll see that the low temp viscosity is quite a bit different while the high temp viscosity only has a few points difference. While I have my own take on this, opinions vary, especially here.
I've attached an analysis of un-used Mobil 1 15W-50 just for entertainment purposes.
