So, what makes you think you know more than the engineers that designed the car?

As I stated above, unless the engine is so loose in tolerance that it is making noise or smoking heavily, using the standard weight oil is recommended...

One other thing - For most practical applications, 20W40 and 20W50 are the same thing... The 40 and 50 are it's "lubricity" number, and the 20W is it's flow ability... if anything, you would want 10W40 winter and 20W50 summer...

but you really should be using 5W30 (or 10W30 is a close substitute)...

 

i use mobil1 0w-40 with 175 000 on my sc3

 

No, everything seems pretty normal, especially the way I drive it, flooring it several time a week lol.