very useful info

it's basically right that a 0w-40 is the same as a 10w-40 at operating temp. but if the 10W-40 is a bad quality mineral oil the 10W-40 will die faster than the full synth 0W-40, even if both have the 40-grade. And how many times do you hear that bullshit that a 0W-40 is too thin for the engine, but a 10W-40 wouldn't Look at the numbers and you will see thats bullshit.

its not a problem to reduce the "W"-number, its even recommended, cause the cold start is always the worst thing for an engine (only very very old engines '50s require single grade oil). What will kill the engine is to reduce the operating temp number after the "W".

If your car manual says your car needs a 15W-40, so you can use a 0W-40 or 5W-50 without thought. But a 0W-30 or a 10W-60 could/will do harm to the engine.