Ok, wouldn't we all assume a 5W-30 would be thinner than a 10W-30 at 40C? Especially from the same manufacturer. I am NOT doing this to just pick on Mobil1 or anything, it's the first one I noticed when comparing 40c cst measurements to 5W-40 Rotella.
So, why is the 10W-30 a lower viscosity at 40C than the 5W-30?
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil1_5W-30.asp
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil_1_10W-30.asp
So, why is the 10W-30 a lower viscosity at 40C than the 5W-30?
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil1_5W-30.asp
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil_1_10W-30.asp