According to my 626's owner's manual, Mazda recommends using a 5W-30 oil when the temperature is below 32F, and 10W-30 when the temperature is above 0F. In my area, during the winter the temperature drops to single digits. I am currently using (and I am very pleased with) Mobil Drive Clean High Mileage (synthetic blend) 10W-30, and I wonder if I should use the same grade during the winter months. I was thinking about using Mobil Drive Clean HM 5W-30, but I have a hard time finding it. I would rather not use M1 (I don't want to risk starting a severe leak). My question is: will I be making a mistake by using MDC HM 10W-30 during the winter? I could use Citgo Supergard (I purchased a case of 5W-30 Supergard for my other car) or Valvoline Maxlife 5W-30 during the winter and MDC HM during the summer, but I don't want to switch back and forth between two different brands of oil.