OK I am officially a 0w20 convert...I think... BUT 0w40 does have a lot of benefits. This is just from personal experience with my vehicles, your results may vary.
Pros:
- Much easier starting than any 5w30. I started to notice the difference from 40°F and below.
- Little to no "metal rattle" at high RPMS, compared to 0w20, 0w30, 5w30. It is more of a metal cricket sound than a rattle, but 0w40 heals it. No, it is not a mechanical problem, just the nature of the engines I serviced.
- Same type of rattle/tick never shows up after excessive idling on 0w40. It always becomes more audible after excessive idling with 0w20, 0w30, 5w30.
- Out of 0w40s that I have tried - PENNZOIL 0w40 SRT actually improves my MPG by a hair across the range. Improved the MPG in 2006 Toyota Highlander Hybrid by 3mpg.
Cons:
- Castrol 0w40, Mobil1 0w40, and PENNZOIL Euro 0w40 give a slight MPG hit. Very little, not significant, maybe 1-2mpg at most across the board. PENNZOIL 0W40 SRT DOES NOT have that issue.
No other Cons from my experience in my daily drivers. Results could be different in more demanding applications. Just sharing my real world experience.