This may make you feel better, my E430 oil spec'd is M1 0W40, German Castrol and others that have minimum HTHS of 3.5. I had been using xW20(PP, Synpower, M1) for more than 2 years. The engine is much quieter, acceleration is faster, coolant is reaching operating temperature quicker but at 5-10F lower, gas mileage is up by about more than 3%, up to 8-10% in some long trips.
The coolant was about 6-15F lower when I was passing Death Valley to Vegas in summer at triple digits speed, the engine seemed to have more power when I went up the steep hills at Death Valley in summer, accelerated from 70MPH to 120+MPH was effortlessly with thinner oil.
The coolant was about 6-15F lower when I was passing Death Valley to Vegas in summer at triple digits speed, the engine seemed to have more power when I went up the steep hills at Death Valley in summer, accelerated from 70MPH to 120+MPH was effortlessly with thinner oil.