If you look at the specifications 5-40 full synthetics actually have better high temp viscosity performance than many 15-40's plus offer the flowability for cold starts.
15-40 is not for my engines:
Cold lube oil and ATFAlso full synthetics will not coke in turbocharger bearings if the engine is stalled or shut down at high exhaust temperatures before the turbo materials are allowed to cool out.
Like "low fat" advertising, there is apparently a bit of fudging in what you can label as "synthetic" oil.
