slant 6 oil weight

It's not about what you like, it's about what you need. Not every engine needs the same oil. That's why they make many variations of oil. I use light oil (0w-40) because of the pressure my car makes. My last car ran 20w50.

If you want to see the benefit of light oil switch from 15w40 to 5w40 in a 7.3l or 6.0l diesel in colder weather.

If you want to see the benefit of heavier oil switch from 20w50 to 0w in a full grooved main bearing engine or something with loose tolerances.