Just dumped in seven quarts of 5W-30 Synthetic into my '01 CLK 430 and I'm beginning to wonder if I should pull it back out and go to something heavier. The hood is still up and frankly I don't want to shut it until I know.

 

The engine will be okay with the lower temp end of 5W 30 I would think.
But as your in the US and I cannot beleive how cheap oil is, just drain it if your in doubt and put in 0W-40 mobil specified oil.

If it makes you feel better, it costs me £50 to buy oil for my CLK 230 which is about $77 !!

 

Thanks for the advice. I was more concerned about the high number and called the local dealer. They said I'd be fine. Closed the bonnet and took it for a ride. The engine didn't explode, so I guess I'm good to go. BTW, nice motor!

 

I would not hesitate to run 5W-30 in winter time. Engine oil is spec'ed based on what the temperature range is climate wise, and what sort of clearances that the engine was manufactured with I think the main thing that makes oil viscosity important in newer engines is the variable valve timing system.