Hello, new to the forum. I'm currently using 5w-30 synthetic oil and I recently bought a bunch of cheap oil. I was able to buy Mobil 1 0w-30 AFE 5qt jugs for $10 each and Valvoline VR1 10w-30 for $3/qt at my local Autozone. I've been told different stories on when to use the oil. Been advised to use 10w-30 in the winter and 0w-30 in the summer and vice-verse, now I'm just confused. What oil should I use and when? Thanks in advanced 1. 2005 Toyota Corolla 1.8L 1ZZ-FE 2. Manual reccomends 5w-30, API Cert: SL, ILSAC 3. New Mexico (the car has seen 8F - 103F temperatures, for temperature trends: http://www.usclimatedata.com/climate/albuquerque/new-mexico/united-states/usnm0370) 4. I drive hard and fast 5. City/highway, equally, ~20 miles a day 6. Short ram intake, running lean EDIT: Car is not on warranty, 120k miles. My oil comes out dark when I change it no matter what oil. When I used conventional oil, I would change it every 3.5k miles; I change it every 5-7k miles on synthetic.