10W-30 / 10W-40 motor oil comparison

When should you use 10W-30 motor oil and when should you use 10W-40 motor oil, in a passenger vehicle gasoline engine?

Also, would 10W-40 leak less than 10W-30, in the same engine?

your car's handbook should tell you exactly which oil to use.

Honestly you could use either and never know a differnce. The differnt weights corospond to what you should use for differnt tempatures, but those two are so close that it would pretty much never make a differnce.