Are you comparing kinematic viscosities at moderate shear rates (ASTM D445), or are you comparing High Temperature High Shear rates (ASTM D4683)?
Two oils that have very similar kinematic viscosities at moderate shear, say 14.1 cSt and 14.3 cSt, may behave entirely different at HTHS (One million shears at 150 C): say 3.9 mPa s and 3.1 mPa s.
A 10W-40 and 10W-30 regular oil are refined from the same light 10W base stock, with polymeric additives to "build" the viscosity at operating temp.
You have to watch out for excessive cross-grading of conventional oils, which implies heavy polymer content. When exposed to a HTHS environment, you can expect permanent viscosity loss.
The Mobil 1 0W-40 is considered a "light duty" 40 and folks here have commented on its somewhat questionable HTHS values: 3.6 mPa s @ 150 C for VOA. At the same time, it's factory-fill in Porsche, BMW, and Mercedes cars, so it can't be that bad.
Mobil Delvac 1 5W-40 is a HD diesel oil cross-graded to API SL for gasoline engines. It has excellent HTHS stability ranging from 3.9-4.0 mPa s @ 150 C and can handle a lot of trash in the oil.
If you're really worried about HTHS protection, perhaps you shouldn't be looking at a xW-40 anyway. Mobil 1 15W-50 is easy to find and appears to have the best additive package of all the Mobil 1 grades. In so cal I don't think you have to worry about MRV BPT too much.
Mobil 1 15W-50 has a kinematic of around 17.4 cSt @ 100 C, with a HTHS of 5.11 mPa s @ 150 C. These are VOA values.