I just found out after owning a 1997 geo prizm with now 80,500 miles on it , I bought it in 2001 with 20,000. That it is supposed to take 5W-30 oil. The original oil cap and service manual were gone. So all this time for the last 8 years or so, I have been changing the oil with 10w30 winter and 10w40 summer..So it light of all of this...have I done anything wrong or that I should be worried about??? PS...never had one problem with the car,,knock on wood...does burn a little oil about 1/2 qt. or so per 3000 miles. mainly use valvoline dino or quaker state dino.