well i glanced at my service manual over the weekend and saw the oil vicosity/temperature page.

basically, it said to used 10w40 where temps are consistantly above 32 degrees. the chart went down to say running straight 10w at temps below -10 degrees. thats sounds crazy.

I didnt see 20w50 listed but f i take all the oil and temps they did list, it sounded like 20w50 might be best if its not used at temps too low. ie, if 10w40 was not recommended for under 32 degrees, I would venture to guess that 20w50 would not be recommended for use around 36-38 degrees or below