Mas malapot yung 5W-40 oil kapag mainit.

"5W-40" is a viscosity rating. The higher the number, the thicker the oil. The "W" means "Winter" so that's the viscosity of the oil when the engine is cold; the other number is the oil's "hot" viscosity.

More viscous oil gives your engine more protection, but at the expense of some fuel economy. Conversely, less viscous oil gives less protection but helps fuel economy.

Maghanap ka na lang ng 5W-30 oil, I imagine your big V6 might guzzle marginally more fuel if you did use 5W-40 oil. Technically there should be no other problems in using it though.